I thought that I sort of implied that in the root post, although I expect that there's more to it than your example.

It's easy these days to get arrested for minor deviltries (way easier than when I was a lad) so the police have that in their pocket.

So, you pick out the groups who are statistically most likely to commit real crimes, pick them up for any old thing, and warehouse them until they are more old and tired.

Like I said, it's not fair, but it appears to work to some extent.