I think it would have been a defensible error if it had been in the moment but the replay was unmistakeable and it happened at a crucial time in the game. Garnett's hand was clearly directly in front of Pau's. To my recollection, Garnett started behind Pau so I'm not certain how he would have gotten the rebound or how Pau committed a foul there. And I hate the Lakers and love it when fouls are called on them.
To be honest I didn't think it was particularly obvious. He went pretty close to straight up and there wasn't any clear arm contact initiated by Kobe. I thought it was a good no call with the Celtics taking the ball out.
  
I think your anti-Lakers bias might be getting the best of you here (and I think all of us have an anti-Lakers bias). The reality is that both teams had the same number of fouls called on them and the Lakers primary front court rotation (Pau, Bynum, and Odom) had 13 fouls called against them while Boston's primary front court rotation (Garnett, Perkins, Sheed, Big Baby) had 17 fouls called against them. And Odom got 5 fouls in 15 minutes! That's even worse than Big Baby's 4 in 18 minutes.
 It's roughly 4 fouls per player on each side, while the Laker's back court had significantly more foul problems than the Celtics. There's a reason Kobe only got 34 minutes in this game.
Now that we can agree on. That was a bad call because the block was completely clean.