b.c the MacBook line features core M processors and are fainless. It can't really be called an Air b.c it is bigger than the MacBook.
They really should have done this:
MacBook Air: Current 12"
Macbook: 13" No TB
MacBook Pro: 13" TB
That requires renaming the MacBook though. IMO that makes the most sense though. I know they want people to buy the TB version, but people will see entry level Pro and jump all over it.