The comments on the Facebook thread are spot on. The city's actions ... not so much.
I sure hope that the owners of every single one of those bikes went through their due process in court before the punishment (vehicle forfeiture) was meted out and that the punishment was not disproportionate to whatever the offence was.
Civil forfeiture has been abused by the authorities every time it has been allowed to happen; there are plenty of horror stories.
edit: A good point was made in one of the posts on Facebook. It's likely that a lot of these bikes were stolen (it's easy to steal a dirt bike, unfortunately) and ridden on the street by the person who stole it, and then the city couldn't be bothered to find out the truth, track down the owner, and return their property.