You've struck a nerve...
There isn't any law that forbids Dish Network or DirecTV from providing you the Atlanta local channels. However, these steps need to occur, in order:
1) The local channel in question must contact all copyright holders on the programming they broadcast, and get a contract to clear the copyright into any and all areas. This means for the Atlanta CBS station, they'd need to clear any and all network programming by contacting CBS and getting the appropriate clearances. (Hint: CBS of course has affiliate agreements that forbid the resale and redistribution of programming, so this step will never be complete because CBS will never agree to it)
2) The station must then sign an agreement with DirecTV and Dish Network, and determine the appropriate coverage area. (Can you imagine setting up 1500 different local channel coverage areas?)
Or you can simply use the copyright exception contained in the laws to have a channel carried only in its market (which is how Atlanta subscribers currently receive Atlanta locals).
The reason you cannot subscribe to Atlanta locals while in another market is because the networks want it that way.