I've only read about the play, haven't seen it, but it sounds like it was a fast bit of action. Its just like a soccer player touching the ball with his/her hands or a baseball player sliding into home. The contact was unintentional and it didn't get caught. 'Cheat' is a strong and unfair word for UNINTENTIONAL contact. Could the Serena's opponent have said 'yes, it hit me'. Sure, but nobody, including Serena herself, would do that. The same goes for every other sport. You take the little breaks that you get. The media may be blowing this out of proportion, but to use the word 'cheat' for something that 99% of athletes would do shows the lack of class Serena Williams has.
My point is, almost every athlete says 'I didn't get tagged' 'I didn't step out of bounds' 'I didn't commit that foul' or whatever. That's part of the game. Those close calls in the middle of fast plays, you don't admit to them if the refs don't see it.