It doesn't seem like a false category to me. "Bugs" to me are cases where the software behaves in a manner directly opposed to how the developer expected when he/she wrote it. UI flaws like this one are cases where the software behaves in a way contrary to how the user -wants- it to behave (in a UI context) but not contrary to how the developer intended.

They both should be fixed, and I agree that using the distinction to pretend UI flaws don't need to be fixed is irresponsible, but I think it's still a valid distinction to make.