Is it always racism?

[Belacqua]

If a certain segment of the population disproportionately commits more crime, then it follows that they'll be disproportionately arrested, unless there's something fishy going on.

Well, I think something fishy is going on.
It's true that "a certain segment" of the population is disproportionately picked up, prosecuted, imprisoned, etc. This isn't proof that they are in fact doing more crime. It may well mean that the system is less likely to give them a break, or a policeman is more likely to make certain judgments. 

If there is racism, then black people will be arrested more and will show up in the statistics as disproportionately criminal. So to show that black people actually do commit more crime, you'd have to show that there's no racism. 

The biggest crimes in recent years have been committed by bankers who got away with it. These were disproportionately white.

And for the record, I don't consider myself a liberal. I am left wing. These days US liberals (Clinton, Biden, et.al.) are not left wing in the least.