I don't know if anyone knows exactly what Bob is doing, but at a stab, he's seeing how many unpleasant feelings get generated by imagining the crime, then proposing a jail sentence that activates about an equal amount of unpleasant feelings. If the thought of a homeless man makes images of crime more readily available and so increases the unpleasant feelings, things won't go well for the homeless man.

To defend poor Bob for a moment, it's worth noting that we don't respond to numbers well in a vacuum. A theft involving a hedge fund manager invokes a frame in which a million dollars isn't that much. A theft involving a homeless person invokes a frame in which a thousand dollars is a lot. I suspect that this magnitude distortion explains more of Bob's behavior than general negative affect towards homeless people.

ETA: Both to mitigate that annoying LW effect where the top-voted comment on an excellent article is always a correction or quibble, and just because it's plain true, I should add that I'm thoroughly enjoying this sequence, and that my rate-of-checking-LW has risen sharply over the last couple weeks since I've been looking for another installment.