Check on that in a dictionary of repute.
My own folk etymology of this infelicity is that it started with the mispronunciation, which is actually hard to avoid in rapid speech, and bled over to people simply writing the wrong word.
Edit: [in reply to your edit]
It is indeed a rather common malapropism.
The OED says that the "house or building..." use of "premise" actually comes from an earlier legal meaning ("The subject of a conveyance or bequest..."). Even for those who (inaccurately) think etymology determines "correctness", this isn't an incorrect use of the word.
I think I'm going to keep on spelling it the way I did.