Jango Fett is the original host for the clones and the cloners keep him there because they need fresh DNA every now and then. On top of that Lama Su explains that Sifo-Dyas picked Jango himself, so it would be odd to do that and then ban him from going there.
Lama Su: We modified their genetic structure to make them less independent than the original host. As a result they are totally obedient, taking any order without question.
Obi-Wan: Who was the original host?
Lama Su: A bounty hunter called Jango Fett. We felt a Jedi would be the perfect choice, but Sifo-Dyas hand-picked Jango Fett himself.
Obi-Wan: Where is this bounty hunter now?
Lama Su: Oh, we keep him here. After a few hundred thousand clones, the genetic pattern starts to fade, so we take a fresh supply. He lives here, but he's free to come and go as he pleases.
Star Wars: Attack of the Clones