A digression: if the government wants to do something, they'll do it. The U.S. constitution didn't stop warrantless wiretapping by the NSA, and if you want to cast the whole Wikileaks thing in the light of free speech they arguably compromised their values there. However, I recognize that the principles of these laws mean a lot, hence this being labelled a digression.
With regard to using my own voice to fight hate speech, I do indeed do that. It's not like these things go straight to the authorities. Here's the problem - their speech is so powerful, so loud. It's so easy to slip into hating people and scapegoating them. What do I tell the person who was rejected from college? "Sorry, guess you aren't up to snuff"? Contrast that with hate groups. They have a whole arsenal lined up on the evils of affirmative action & quotas and how that potential student is the REAL victim of the system. Which is more appealing at face value to the potential student?
On the whole not arresting someone = giving them a platform thing: I previously mentioned the principle of having laws. In Canada people know that there is opposition to racism at an institutional level, and I believe that is very powerful. I also believe that if you have the means to stop an atrocity, and you do not, you are complicit in its execution. This of course is a moral problem for the ages, and given the seemingly high number of objectivists on HN I doubt it's very popular here.
On the evolution of hate speech laws: I fully expect transphobia will be covered under the hate speech laws before long. Where is the line? Not sure. There's a whole bunch of stuff which some people claim lies further along the social justice "axis" (and which has sometimes been mercilessly lampooned[0][1] by the SJ community itself) which I don't really agree with, but... well, who knows in 50 years. I have a great amount of trust in the Canadian government and our judicial system, so I'm sure our judgment will carry us well into the future.
[0] - http://questionablecontent.net/view.php?comic=2229
[1] - https://twitter.com/tumblrtxt
edit: I forgot to address your core point! "I'm going to throw you in jail because you believe something and then you said it." In a vacuum this is of course ridiculous, but we aren't in a vacuum. The simple answer is that hate speech can cause a great deal of harm to society, and I support laws preventing this.