Of course he does, the provided him training facilities and coaches for years. He cannoy just leave when he becomes an okay player when he has always had monetary support from the USA.
You really love to complain, did you even read the article? It does not say that he cannot play for Russia, it says he must repay the USA for their help throughout the recent years.
You can easily liken this story to a story in many corporate places where you cannot jump straight to a competitor.
He is top 35, its a big deal, precedent setting really.
How much would he have been in debt if he were given nothing? If a player is given $100k but spends $140k, does he owe nothing to the USA in that situation?
However put it this way, if he was representing the USA now.. what would the USA be gaining from him? They really obviously havnt given a shit about him at all for the last x years until now he has a decent ranking and changed country to represent in the davis cup. Should Sharapova also represent the USA in that case?