There is no legal reason to ban Poker HUDs on any poker site. That is why, majority of poker sites such as 888Poker, Winamax, and iPoker network allow HUDs on their tables. For experienced players, Poker HUDs make the game more competitive.
If you were only wondering if online poker is rigged, the firm and final answer is no, the game is not rigged, and countless pieces of evidence support that claim. However, this does not mean you are perfectly safe when you play online poker, as there are other things you need to be careful of.