Ok, then for curiosity's sake, why do you accept it just fine in the psionic powers category for fantasy?
I agree. However, I think what you mean isn't so much less fantasy as more grounding and rationality in the world building. Whether you call them psychic powers, they're still magic and the difference is in the presentation. Even a generic fantasy setting could appear like scifi if you present it as such.
For example, psychic waves can be produced with technology such as psi-emitters and psi-disrupters. In fact certain applications, such as psychic jamming, are only feasible with industrial engineering.
The psychic powers are still subject to Sanderson's laws of magic (a writing technique). Psychic powers need some kind of logic behind their workings or predictable limitations otherwise they risk becoming a deus ex machina. IMO, an easy way to do this is to introduce realistic arms races as an important plot point: whenever one side develops a new weapon, the otherwise develops a countermeasure, ad nauseum.
Deus ex machina is a classic example of bad writing.
I said plenty of times that the writing of SC1 and BW wasn't great because Metzen was clearly making it up as he went along. Rebel Yell had the strongest writing (although it still suffered from the psi-emitter being a blatant plot device), but after that the alien campaigns fell apart.
At some point you'll write yourself into a corner (however small) where you can't really explain it away. Even you have to have seen sci-fi novels (and I'm not talking the SC ones) get into that situation.
You can argue it that way, but not everything is meant to be explained away via logic. Granted, too much deus ex machina is also bad....
That sort of attitude will not get you very far in the literary world. The literary world universally agrees that any amount of deus ex machina is bad because it removes all tension from the story and destroys the audience's suspension of disbelief. But I digress.
My original point is that the Starcraft story is shlock and no amount of contrived excuses will change that. Blatant retcons, maybe. Amon is probably the least of the story's problems (the heroes being sociopathic morons is arguably worse), but I'll focus on him in particular here. You keep talking about how the Amon backstory makes sense, but you never provide any explanation. You have never once shown that his backstory can be salvaged. Somebody else has beat you too it.
Lord22 on fanfiction.net has written some adaptations of the Starcraft game campaigns that, among other things, try to reconcile the retcons with more retcons. The reconciliation for the Amon retcon? The Overmind was never under Amon's control, because his attempt to control it failed and it exterminated the xel'naga in self-defense. Amon wanted to replace the xel'naga with his mindless slaves, the hybrids. The Overmind wanted to create a new generation of xel'naga who would be even cooler than the last and would replace the previous infinite cycle with a Mass Effect-esque harvesting cycle.
It is a decent attempt at salvaging the canon plot outline. I still dislike retreads of the canon plot outline because it simply was never very good to begin given the immense possibility of the premise, but I have to give Lord22 credit for the work done so far. Also, his critiques of the canon plot are hilarious to read.
I never read from Lord22 on ff.net
As for the whole salvaging, last I checked you just wanted to smash the whole thing and start over from scratch anyway