User blog comment:LadyMaryBryan/Cullens Face Real Consequences/@comment-38474921-20190210223510

The rule is not that a human is forbidden to know; it's that when a human discovers the truth, one of two things must happen:

1.) the human shall be killed, or

2.) you must have the full intent to transform the human and begin your preparations to do so

The Volturi themselves even tend keep a 'pet' human around who knows the secret. This is because thier servant fully understands the consequences of disclosing this knowledge with anyone, and they will eventually turn her if they decide they want to keep her around. Otherwise they will just kill her.

The reason for their not requiring Bella's immediate transformation is because they do understand the complications involved. Her needing to be inconspiculously pulled away from her human life is their particular situation. They know it isn't best to yank her away from her family suddenly. In general cases, a vampire might find a human whom he or she wants to make a companion or mate of. But not every vampire has the instant willpower to do this succesfully. It takes an extraordinary amound of mind-over-matter strength to initiate the transformation without killing the human. So there may be a bit of a time where the vampire must get accustomed to being near the human and really decide whether he or she can do it.