Removing item from cart does not cancel registration

Create a registration. Remove it from the cart. The registration still exists, with status of pending.

User decides they really do want to register and tries to add it to the cart - but they can't since the registration already exists.

I would expect removing registration from the cart to automatically cancel the registration (and I'm assuming if the registration is cancelled, the registrant would be allowed to register again.

Ideally, this would be in the code, but I'd be happy to have a reaction rule to do it.