We seem too ready to forget that the cancer/tobacco link was itself once controversial.
1. Tobacco itself is quite carcinogenic. Snuff, chew, gutka, and any other means of consumption all have strong cancer links. Cannabis, as I understand it, is not nearly as carcinogenic as that.
2. Smoking is also, separately, carcinogenic. Regularly breathing dense smoke is terrible for your lungs, even if you're smoking dried lettuce.
I think it's pretty well established that via 2, smoking cannabis is bad for you. If it's less bad than tobacco, that's as likely to be usage habits as anything. But people are still arguing fairly extensively over 1 - tobacco is a known carcinogen no matter how you ingest it, but I'm not sure that's a settled question for cannabis.
If by "usage habits" you mean "usual quantities of material smoked and inhaled", then, sure, that's probably a pretty big difference. Now, legality (and, price, which is strongly influenced by legality) plays some role in that, but the inherent nature of the effects of each drug and their concentrations in the material that is smoked also play a pretty big role.
But really, no one should be inhaling the smoke from anything that's on fire. Nor even eating food seared at high temperatures or by open flames. Some of those Maillard, caramelization, pyrolysis, and carbonization reaction products (like acrolein, acrylamide, heterocyclic amines, and polycyclic aromatics) may contribute significantly to flavor, but they are also very bad for your body, and are similar, if not identical, to the chemicals produced in smoked plants.
So you don't even escape that risk with cannabis edibles, because as I understand it, the plant matter has to be heated above 100C in order to produce the desired recreational effects, and some (impatient) people accomplish that by grilling it in a skillet, destroying some of the potentially medicinal molecules and introducing carcinogenic ones. This is why you really need someone trained in proper lab technique to test the claims. No two amateur stoners will prepare identical cultivars of cannabis in exactly the same way. The cannabis researcher will isolate variables and devise a repeatable experiment.
It is unfortunate that we have little reliable evidence to confirm or deny the anti-carcinogenic cannabis hypothesis, just because of political interference with the science. Few reputable scientists want to touch the stuff, because the DEA makes it such a pain, and there is also the risk of persecution resulting from politically unpopular findings. It would be really, really embarrassing for the US federal government if anyone were actually able to prove that whole cannabis treatments (rather than just a single purified component, like Marinol) either reduce cancers directly or reduce the negative side effects from proven cancer treatments.
Cannabis has been shown to have anti-cancer properties in at least two clinical studies that I know of. Not the miracle cure that some cannabis advocates say it is, but certainly worth researching.
The fact that cannabis reduces side effects of cancer treatments is well known and a major reason why it is prescribed in the US.
It's absolutely ridiculous that the DEA continues to insist that it's not a medicine, it's clearly a medicine.
This is why our smoking/cancer statistics reflect a reality in which it was once extremely common for smokers to be inhaling one or more ounces of burned tobacco per day.
Most people already know that that's a huge amount of weed for an average person. Even out of folks who are getting high every day, almost none are hitting that mark-- the most obvious proof of that being that it would cost them hundreds or thousands of dollars per day. Even if we allow for unfiltered cannabis smoke to be several times more carcinogenic than commercial cigarette smoke, we're not going to get there.
In fact the opposite is true; the average person who develops a serious weed habit (more than a few grams a week) is actively incentivized to try safe methods of consumption like vaporization, extracts, or edibles, if nothing else because they will save a lot of money.
So apples-to-apples, we wind up comparing the habits of generic cigarette smokers to those of drug dealers and very wealthy, very pot-addicted people. I understand the desire not to approve of any amount of smoke inhalation, but the null hypothesis here is not equivalent risk.
Apart from all of that, our generation didn't invent smoking weed, and neither did our parents. If cannabis represents a public health hazard on a similar scale to cigarette smoking, it's hiding pretty well after all this time.
Still, after 80 years, how are you splitting responsibility for that between our laws and culture and the substance itself? Smoking cigarettes furtively is much more difficult than smoking weed furtively[0], due to the exact same two factors I pointed out previously.
If the same evidence that predicts you'll see less disease predicts you'll see fewer of the users, does not seeing as many users really count against the prediction of less disease? At the very least it seems like you'd have to assign some priors in order to know which way that evidence points.
[0] In the habitual sense, obviously.
There are other things you can mix with water and use an insecticide, too. Caffeine, for one. There are other reasons that we don't use it, but mostly practical ones (it washes away easily, it also kills things in the soil).
Because most studies showed that tobacco didn't cause cancer up until cigarettes were invented. And even to this day it's still not really clear to what extent tobacco itself causes cancer.
Nicotine is a nootropic
no signs that it causes cancer
Its a hard connection to prove, but all smoke contains chemicals that are known to cause cancer. Eaten cannabis probably isn't carcinogenic, but smoked probably is. Few cannabis smokers inhale nearly as much smoke as tobacco smokers. Also, tobacco is a nightshade and toxic if eaten, so maybe that has something to with it being more damaging when smoked.