The academy, which Biggs had already graduated from, was presumed to be some sort of civilian service, although the specter of being drafted by the Empire hung in the air. Which is why Biggs was going to jump ship and try to find the Rebellion. Luke was the only person he told of his intentions.

Luke was clearly interested in hearing about the Rebellion from Threepio. And he clearly told Ben he hated the Empire.

Stealing an X-Wing is a bit of a stretch. The fleet might be scattered across the galaxy for a time before we saw them at the end of ESB, in order to evade the Imperial fleet. In light of the fact he blew up the Death Star without a targeting computer, saving the Rebellion's collective ass, I think the leadership would cut Luke a little slack if he went somewhere to improve his abilities.

They might dock his pay for leaving his X-Wing on Bespin though. ;)