Shasa was a recovering addict who had turned to crime in order to buy Spice. But she served her time, and was released to Jabba's Palace/halfway house. As long as she stayed there, and submitted the weekly urine tests, she was free. If she ever stepped out of line, Jabba himself would have made sure she was turned back in.
Shasa had started to get a little down by the time Luke Skywalker showed up at Jabba's Palace. The monotony of regular life was starting to get to her. It was a little exciting though to see the Wookie push that actor jerk down the stairs. And Luke fighting the Rancor was compelling. She was even happy to go out in the open air on the Sail Barge. Of course, she was never bored again once that maniac Skywalker blew her and everyone else up on the Barge.