Because they get all the breaks from the refs, and are known for playing dirty. Not so sure they still play dirty all that much, but they still get breaks from refs. Basically the hatred goes back to 2006ish when they played the Suns in that epic series. I think it was Robert Horry who throws nash into the scorer's table. A brawl ensues. The suns get 2 players suspended, the spurs get 1. Spurs go on to win the series. Of course that was a while ago so my memory is fuzzy
Then fast forward to this past week, Manu travels the other night before making the game winner, ref swallows the whistle, which I didn't mind too much. Then the next game Manu slides in front of Carmello whos throwing up the game winner, and the refs call the charge.
So basically they've always been a bunch of dirty foreign pricks. Even their coach is a dick...although he is the second best coach in the league.