[premisis]

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.

[BalinKing]

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.

[hollerith]

I think I'm going to keep on spelling it the way I did.

[premisis]

You can do that. And, for some percentage of the population who reads your writing, you will appear under-educated in this matter. It will look like you dont know the right word to use. The other part of the population won't notice.

[alsetmusic]

And a very small percentage of the population will need to be right so badly that they point it out on HN. C'mon, I'm super pedantic and even I think you're going a bit far. Also, no need to throw shade when pointing out grammatical and spelling issues.