Arena is a lot easier to beat than NFL. Linesmakers simply don't have a clue, and base it all on perception. In NFL, the public might move a line a point or two from where it is, but Arena lines were routinely 7 points off (and while the averae margin of victory was slightly larger in Arena, it wasn't that much higher).
Not all books were taking Arena action last year. Being on NBC all season really helped the Arena league, and now most books offer Arena lines. This year, NBC will televise more game, and there are 3 expansion teams (for a total of 19). It's a perfect substitution for NFL during the spring.