You still don't get it. Its simple. The referee's (I am one - I have worked HS basketball for 15 years) main responsibility is to 1) enforce the rules, 2) keep players safe, and overall, see to it that the game is decided FAIRLY.
"Fairly" means, simply, that when one player bends/violates the rules such that her team gains an UNFAIR ADVANTAGE, the referee is obligated to intervene with a call. As has been articulately explained by umpteen other posters already (and you've said yourself it was a foul), that screen play gave UConn an unfair advantage.
"Let the players decide the game. Not the refs." The player (screener) DID decide the game. She clearly violated the rule, gave her team an unfair advantage, and the appropiate consequence followed by the referee.
If you can't grasp that concept, its probably because of basic disrespect for officials (common on this board) more than anything else. Try officiating basketball yourself, and you'll have a different perspective.