By that logic, should a character be allowed to apply Strong-minded to resisting persuasion tests, because someone with some smidgeon of psychic potential is trying to influence their mind? And, furthermore, if this is is so psychic in nature as to count as a psychic power, why can Khorne-aligned people do it?
No. Someone trying to convince you of something, unless actively using a power to do it, isn't affecting you with psychic powers. You're grasping at straws there. The Daemonic Mastery test is an entirely different matter. Its a sorcerous ritual, in the psychic powers chapters, affected by Psy Rating. Even Khornites are allowed to do rituals and use warp-infused powers and abilities, just not "be a sorceror/psyker". Its hypocritical, but that's how he is. A Khornite just wouldn't be allowed to use his full psychic force, but they aren't blanks: would those even be able to perform it when their mere proximity can screw with the warp?
Or maybe because they didn't want Daemons to add their psy rating to the test. We can guess the motivations of the developers all we want, but at the end of the day you don't, by the rules of the book
Little choice but to try to guess at the motivations when asking whether or not such a thing was intended or forgotten. We already know its not as such in the book. The "effective" psy rating is a good point though, its possible that it wouldn't be allowed as a result. If errata or changes [next game or whatever] happened to allow daemons their PSY, that would definitely need to be answered as a result.
Yet on that table listing the modifiers there is mentioned neither psy rating nor binding strength. And with no overarching rule capping modifiers to 60, you're conjecturing here
Despite its placement, Page 241 seems to be pretty definite. "If a situation calls for two or more bonuses or penalties, simply combine all modifiers together and apply the total to the appropriate Characteristic. The maximum total bonus that can be applied to a test is +60. Conversely, the maximum total penalty that can be applied to a test is –60." Also in Broken Chains page 10: "The maximum modifier that can be applied to a Skill Test or Characteristic Test is +60 or –60."
This ain't conjecture. Its ONE test, with the +/- 60 limit, to which -seperately from the standard summoning because weapons are what have a binding strength to them [and daemonhosts, I know, but they're not in the book]- is added the weapon's binding as an extra penalty. 5-22 is also summoning modifiers there for the purpose of that specific ritual.
You're probably supposed to use those, not the standard mastery test, but the wording has once again been too weak and makes it look like you add both together, making the capturer's +60 more or less guaranteed. The Daemon's not getting binding strength against his will till he's bound in the weapon though. Binding only exists in a completed daemon weapon's attempts to take over.
edit: I'd made a gigantic quote block. That's no good.