They're official 'against interfering' but the series makes clear that the Time Lord are a corrupt race. As the Doctor says in "The Two Doctors" " I'm simply a messenger. Officially, I'm here quite unofficially. I'm a pariah, exiled from Time Lord society, so they can always deny sending me."
Basically, the reason they sent him to avert the Daleks' creation in "Genesis of the Daleks" is because they'll happily break their own rules if they think nobody will ever know.