You can certainly go to any public space and do this, however. People do it all the time (admittedly less frequently with megaphones). Are all of the people on street corners doing twirlies with cardboard signs immoral? Billboards would be a gray area example whereby they're hosted on private resources (land) but intrude into public space (view from highway).
> Imagine if you accept a "free sample" box of cereal and you get home and open it and it's just full of flyers, instead of being full of cereal.
Imagine if you accept a "free social media feed" of information about your community, and you "get home" and it's full of ads. Or you accept a "free article" from a website by clicking on a link, and when you load it (consuming bandwidth on a line that you paid for), it contains just as many ads as it does paragraphs of information.
As I said, I'm not defending spam in general (which is obnoxious), or the act of the person/people who polluted/vandalized the npm repos. I just think "immoral" is a little strong unless you also want to paint much of the rest of the ad world with the same brush.