I'm merely applying the Principle of Indifference and the Principle of Maximum Entropy to the situation
You can do that when dealing with things like coins, dice or cards. It is extremely dubious when one is doing so with hard to classify options and it isn't clear that there's anything natural about the classifications in question. In your particular case, the distinction between altruism and utilitarianism provides an excellent example: someone else could just as well reason by splitting the AIs into egoist and non-egoist AI and conclude that there's a 1/2 chance of an egoist AI.
A 1/2 chance of an egoist A.I. is quite possible. At this point, I don't pretend that my assertion of three equally prevalent moral categories is necessarily right. The point I am trying to ultimately get across is that the possibility of an Egoist Unfriendly A.I. exists, regardless of how we try to program the A.I. to be otherwise, because it is impossible to prevent the possibility that an A.I. Existential Crisis will override whatever we do to try to constrain the A.I.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.