This is a known issue. See correct-draw-claiming and erroneous-draw. It seems there are no easy answers.
{Incidentally, in online chess, chess.com used to implement offering a draw strictly according to the FIDE rule - but they changed it! See offering-a-draw-on-chesscom (and please-make-quotpending-draw-offerquot-easier-to-see).}
How would the system know what move you were about to make? That is why it is after you make the third repetition.
They would have to implement a clunky system where you click draw, click on a button that says repetition and then entered your move. Considering how many turbo chess games are played on this site that would likely be considered too slow so you simply move and click draw immediately.
I was playing a game of live chess earlier today and tried to claim a draw by repetition, but was not able to. It was my move and I was about to repeat the position for a third time, so I pressed the "draw" button, but it offered a draw instead.
Later I looked up "how do I claim a draw" on chess.com at http://support.chess.com/Knowledgebase/Article/View/40/3/how-do-i-claim-a-draw-for-the-50-move-rule-or-three-fold-repetition. It says,
"Once the position has repeated three times, make your move then click on the "Draw" button. This ends the game as a draw."
This contradicts the official FIDE rule that
"The game is drawn, upon a correct claim by the player having the move, when the same position, for at least the third time (not necessarily by sequential repetition of moves)
a. is about to appear, if he first writes his move on his scoresheet and declares to the arbiter his intention to make this move, or
b. has just appeared, and the player claiming the draw has the move."
I understand that there is some gray area when it comes to this rule, but I think chess.com should match the rules of live chess with the official FIDE rules to avoid confusion.