Lets be fair, a lot of people have been abusing these plans for years. Streaming HD Netflix 24/7 or tethering to your computer so you can torrent 50-100GB a month over a cell signal? That's abusing the network and the abusers know it.
Unlimited plans were a poorly thought-out decision by the carriers, from a time where data demands were much lower and that demand rises exponentially every year. They share blame in this situation also no doubt about it, and they should have had the foresight to know their networks were going to get hammered more and more every year as smartphones exploded, but I can't blame them for trying to find ways to get the "heavy users" off of the unlimited plans.
I can't fathom how people could use up to 100+ GB over their phones. I manage to stay under my $80/month, one measly gigabyte limit every month (Telus in Canada). Do people really need to stream Netflix or whatever THAT much in a month? Or do they just do it out of spite, all the while saying "HAHA JUST TRY AND STOP ME" to the carriers?
Again, the carriers should have known better. But on the other hand as long as you keep your usage reasonable you shouldn't have to worry too much about the throttling. I thought I saw an example of a 5GB a month threshold somewhere in this thread and I would KILL to have a limit like that on my cell, and would likely never even come close to using it all.