The FAA can't arrest you, but they can fine you and revoke your certificate if you actually have one. That's the difference between law and regulation, if I understand it correctly.
They are probably arresting him for things unrelated to UAS usage, like drug possession.
I think the "arrest of drone rule violations" in the US is such a grey area that it will create an issue down the road. I also think the FAA is losing ground on a chance to clamp down on careless operation as its apparent more and more violations are out there. Drone violations reports are up in this FSDO region. . . it will be interesting to see what the FAA reports for the year end totals.