The rule is that the player with the ball must slide FEET FIRST. This is a declaration of "giving himself up".
Ginn did NOT legally give himself up. He did not slide feet first. He just caught the ball and sat down. If the other player had not touched him, the clock would have kept running.

Of course, that is a pretty arcane rule that one wonders if Ginn or the D had any idea about.

In any case, the Houston D played it about as badly as it could be done, and the Saints were amazingly fortunate. Including the completion of a low percentage 58 yard FG to seal the game.