To your reasons, I'd add financial interests of those involved in the WoD. Think of pre-trial asset confiscation, WoD-based grants to local law enforcement, $ to aerospace companies for WoD surveillance and detection technology, etc.
Legalization would hit many powerful interests in their revenue streams.
Drugs are also a perfect excuse for anything. Drop a bag of something in someone's car and suddenly they have to prove it's not theirs. They can't, of course, so now anything you do to them is justified.
Did you accidentally beat up the wrong person? Plant drugs on them and charge them with resisting arrest and possession.
Legalization would hit many powerful interests in their revenue streams.