In the end, the guy pleaded guilty not just on the internet but in a court of law.
He's a scared 19-year-old up against a police department that wants to have him up on a cross. His parents (who obviously failed to teach him when it's OK to push your luck and when it's not) are probably siding with the cops to now "teach him a lesson", so he's got no one in his corner.
Put yourself in that position, and imagine the DA (or whatever Canadian equivalent) gives you a choice between pleading guilty and not driving for half a year, or facing jail time if you contest the charge. You'd plead guilty too, even if it IS a gross violation of due process - you don't have the requisite tools/experience to fight the system, and knowing that you were legally right is of little consolation when you're being savaged by your cell mate.
This guy may or may not have been speeding.If he actually was (which I personally do think, but that's irrelevant), there is no way a witness could identify him. At 87mph, you can't read a license plate or even identify the model of the car. The best you can do is say a white BMW went by really fast. If we're really saying that's good enough to convict someone who has a white BMW because, well, he's a teenager and should be taught to slow down -- despite how the law is written -- then we may as well do away with police and courts altogether and go back to lynch mobs.
(Incidentally, if you actually read the thread where he posted his boast, you'll see that after enough people pointed out the error of his ways he came around and agreed with them. Lesson learned. The best education comes from your peers and the people you look up to, not the people you're afraid of.)
As an aside, the snitch in question who thought the best use of his time in the U.S. was to call Canada and report that one of their citizens might have been speeding should be outed and dealt with appropriately.