@girlonthenet I would assume that either it's an offence they believe they can get to stick, or it's an excuse to raid his properties with a high degree of confidence that they'll find something far more damning.
No-one is going to arrest the artist formerly known as a Prince unless they're sure, because there's no graceful climb-down to be had here if they can't secure a conviction. Plus that would send a really terrible message to all the others who think they're untouchable.
IMHO, etc etc.