Because the guy who owns it has demonstrated repeatedly he cannot be trusted and that your API use is entirely beholden to his whims.

If you integrate Grok into your product in a user-facing way, one day it could suddenly become MechaHitler and you wouldn't know until a user told you it called for eugenics.

If Elon is willing to do this to the model they show and give to the public on a whim, he is 100% willing to do it to you.