I don't think there's a specific rule that prevents a player from folding when it is his or her turn.
However, how do we feel about situations where a player can check, but decides to fold? If there are multiple players vying for the pot, I have an issue with this.
For example, suppose everyone folds to the hijack, who limps in. The button raise, and you are in the BB and call. Hijack calls. Flop comes down, you check, and the player in the hijack folds. I feel that this gives the button an unfair advantage over me, as he has one less opponent to worry about when considering whether or not to bet. That is to say, by folding, the hijack has failed to protect all of the other players remaining in the hand.
Thoughts?