Why doesn't Amy have her own TARDIS key yet?

[DAK]

Probably wise, you don't know where they've been...or possibly you do!

That's probably why the Doctor didn't insist that he give it back.

 

[Starkers]

I think we need to stop this line of reasoning right now!

 

[The Wormhole]

What a disturbing thought...

It gets more disturbing. Imagine Jack standing in a crowd. We then hear the TARDIS "vworp" noise. Then everyone else in the crowd covers their noses and tries to stand away from Jack.

 

[David cgc]

Actually, since the TARDIS key heats up and glows when the TARDIS is doing something, that might be noticeable unpleasant.

Or it fell out of his pocket and he picked it up off the ground, being too hardy to be vaporized by a simple defabricator. Actually, come to think of it, hasn't the Doctor met Houdini? He could pass some tips on to Jack. It could've been in his mouth.

 

[Snaploud]

Jack let himself into the TARDIS in "Bad Wolf"; that's how he figured out where Rose went.

In "Vampires of Venice" the Doctor tells Amy and Rory to go wait for him in the TARDIS, so maybe she does have a key and they just didn't make a big production out of him giving it to her? Also, at the end of that episode she just opens the door; the Doctor never unlocked it for her. Maybe the TARDIS just lets in whoever she wants, and the key isn't really needed at all. She did lock the Doctor out in "The 11th Hour".

In The Hungry Earth, Rory goes back into put Amy's engagement ring somewhere safe - so maybe he's got a key and Amy doesn't?

I wondered about that moment. I've also noticed at least one more time in which the doctor walks into the Tardis and lets the door slam shut behind him...only to have Rory and Amy re-open the door (without using any key). Perhaps the doctor is able to permanently unlock the door for a certain period of time...? Although, I doubt he would do so in the situation you mention.

Perhaps the finger snapping thing is the the doctor communicating with the Tardis...perhaps the Tardis has developed the ability to know who to let in...?

 

[David cgc]

Well, he seems to have to explicitly lock the door. I noticed he seemed to do it more often as time went by during the RTD era. Not sure what was up with that.

 

[Stevil2001]

Explicitly locking the door is like a super-lock, I think. At least, when the Master turns the lock in "Utopia," the Doctor can't get in even with the key.