Actually, there's a lot of good reason to use 22s.
Pistols and ammo are cheap, and can be easily aquired and disposed of. The revolvers carry up to 9 rounds, are extremely reliable, and do not leave shell casings behind. The bullets are hard to recover undamaged for tracing, and the rimfire casings have no distinctive extractor/ejector marks. Also they are not too noisy, especially if you use shorts.
It's good business.