It's not so much that tie-in games suck, it's that usually the games are given so little development time and a strict deadline (the movie's release) that the company can't do anything BUT shovel out a steaming turd pile. Given a sufficient amount of time, you can still come up with good games. See: Duck Tales, Goldeneye 007, Spider-Man 2, Chronicles of Riddick (the remake, not the first one), a couple of the Harry Potter games, Batman: Arkham Asylum, KOTOR (does this count as a tie-in, though?)... I could go on. Ubisoft is just stupid to assume that a movie license is guaranteed money just because Harry Potter games sold well. Well, of course Harry Potter sold well! It's a quality children's movie, and parents buy their kids video games! Meanwhile, Avatar has a far broader audience with members who probably aren't even aware there is a tie-in game and likewise, don't care.
Also the game looks pretty bland and uninspired from the screenshots, which is ironic when the movie had such a grand vision of its world, to the point where it was the saving grace of an otherwise giant cliche storm.