I believe God let the witch call up Samuel just because it pleased Him to do so. Plus it was Saul, the King of Israel committing this abomination. Even though the Lord had left him, and He had left him because of some unpardonable sins, usurping the priesthood and trying to be a prophet, not the least of them, Saul was still Saul. (David put to death the man who slayed him).
I don't think any demon could have given that information in such detail to Saul either, and that the knowledge of all that was about to happen had to come from God, for God Himself was about to bring all that upon Saul.
All very good points. It certainly is very possible that it happened just that way, and the plain reading of the passage would more support your interpretation. It does portray it as a conversation between Samuel and Saul. It is also a reasonable point to make that Scripture could have stated that this was a demon, and didn't.
With regard to the predictive information given, I wouldn't regard that part as proof. While the demons live inside time, just like us, and cannot see the future, they have had thousands of years of experience in dealing with us, and often can make some pretty "educated guesses" in an attempt to deceive and destroy. Also, as I pointed out, a prediction like that one was a bit self-fulfilling given the effect it would have on Saul's state of mind and morale.