Compulsory helmets just for cyclists is a bit arbitrary too, since pedestrians and car occupants would also benefit from helmets, and at least pedestrians are likely to encounter the same exact crash scenarios which cyclists encounter where helmets are necessary (e.g. pedestrians can also get hit by cars and flung a distance)

It looks like new straps cost ~$8 on Amazon, so the helmet is worth at most $12. Which isn't nothing, but... unless you happen to need a helmet and the helmet just so happens to be the right size for your body, you'd need to sell it, and no one is going to pay full price for a used helmet. I'd be amazed if you got more than a couple dollars when all was said and done.

I suppose you could buy straps in bulk for much less money, but then you're basically setting up a helmet-stealing operation... it all just seems like so much work, for very little gain! If you're that desperate, wouldn't it make more sense to invest in wire cutters so you can steal the bikes themselves?

I agree that it's odd that people would steal these individual pieces, but it does happen. It reminds me of people only stealing 2 rims from a car.

I've been in areas with large drug problems, and people steal weird things (and avoid stealing less weird things) all the time.

Also, generally speaking the more valuable things are the riskier they are. It's a lot easier to pawn some random bike helmet, but bikes have registration numbers, and you may need to enlist the help of a chop shop that's in on what you're doing.