I went to
this blog to read about the red cards.
Interesting range of opinions, but no resolution. Some say the U.S. players should know they'll get red cards for sliding tackles from the front. But others say the U.S. team is the only one in the tournament that got straight red cards for those plays.
To my eyes, the play against Spain was a straight 50-50 -- two guys going for the ball, with incidental contact. Just the fact that the U.S. was winning 2-0 at that point seems like an argument against the play being intentionally reckless.
But, like I said, I'm new to this. I'm just trying to figure out what was so special about that tackle to merit a red card, especially in the context of that game.