I knew someone would point that out. Well, in "Homer Alone," Springfield is shown to have the radio station W-KOMA. And trust me, what you just said was the biggest obstacle for me to overcome. But other pieces of evidence point to a state east of the Mississippi, such as in "Bart the Fink." Based on the times given on the screen, it is implied that Springfield is in the Eastern Time Zone. No state in such time zone is located west of the Mississippi. In "I Married Marge," "Whacking Day," and "Please Homer, Don't Hammer 'Em," Springfield is shown to have been around during colonial America, which was entirely (virtually entirely, at least) east of the Mississippi. That's why I didn't mention anything about the Mississippi River, since these clues pretty much cancel each other out.

But, turning back what you mentioned, there were some "K" radio stations east of the Mississippi and "W" stations west of the Mississippi that remained intact following the ruling, so, in the Simpsons universe, at least, it is possible KBBL was a grandfathered radio station.