the actions of police officers are not rational. many cops are willing to arrest people they don't like just to give them a hard time, even when they know the charges won't stick. cops NEVER get punished for this kind of thing. some cops just like making a hippy spend a night in jail because of the discomfort and inconvenience it causes to the detained person. don't be surprised when a job that grants authoritarian privileges attracts authoritarians with a sadistic streak.

'You're under arrest for having murdered in the past even though you have no murder weapon on you!'

I'm not at all in favor of criminalising recreational drugs, But arresting someone for having done something illegal without them currently doing said illegal thing makes sense.