Comment by Four eyes on One Piece - Chapter 1166

@Four eyes Maybe bc Dragon was not a pirate but a marine who went against the marine laws but the guards couldn't care less if he escaped bc he was still unknown at that time
That's why Garp could easily let him out. 
And Ace arrest was already known by the world and he's the son of the former Pirate King, and was already a big shot of being in the Whitebeard pirates crew so his surveillance was high so Garp can't do anything abt it