Why after 12:00 PM?
What has a time of day to do with anything?
Because most religious groups that have Sunday for their sabbath are done with their services by noon.
If nothing else, anyone who remembers the description of Pearl Harbor should be able to see a certain unspoken respect for those who choose to worship. Even if one doesn't chose to do so for himself, he can respect others who do.
Sure, banning sales on Sunday morning won't guarantee preventing a drunken sot from bothering church folks, but it's a good deterrant. That's what laws are for. We all know there are no guarantees and that's why we have deterrants.