"Know" isn't really the right word there, unless you're describing your feelings or this is just rhetoric. A Champions character isn't allowed to wear a defeated enemy's magic ring, but that isn't because he doesn't know how to wear rings.
Did they apply their training to learning to equip the ring? You see, this has very little to do with what the skill trees bring to the table. In my karate class, the first two White Belt techniques had a total of 8 moves. By the game's definition, that would make a novice student a black belt, since they've learned all the different moves they can use in a match? Did you see the Karate Kid movies? Assuming yes: I fought in those kinds of tournaments. If I had gone in limiting myself to 8 moves, I'd never win, no matter how they were applied, because my opponent isn't limited. Assuming equal skill levels, the artificial limitation of 8 possible skills means that he has a larger pool of abilities and counters to choose from, and so, I lose.
To apply this to the game, the skill trees are technique lists. Limiting what a character can use off the list, once they've learned the technique means that you have to imply that, during combat, they "forget" things they know. This is artificial difficulty, since it's artificially limiting what they can use in a combat situation because "UI Limitation". Is it because people can't access more skills than that? Certainly not on the PC, since Ctrl, Shift and Alt can all be used to access different bars, and this tactic was applied, by BioWare, in NWN. My Xbox controller only has one modifier key, by default, although I suppose I could make more, but the removal of the Radial Menu for skill use effectively limited what consoles can do. So, even though I'd like to believe otherwise, I have to start looking at "balancing between platforms" as a reason for this, which has absolutely nothing to do with gameplay, other than some platforms have more access than others.
Is this a straight-up premise? If not, could you work out the derivation?
I think the analogy above sort of touches on this: I have trained sufficiently in whatever school, be it warrior, rogue or mage, to learn specific techniques/spells/talents. I come over the hill and run into a mass of enemies, the talents/spells/skills I have on my hotbar are "largely" ineffective, but I can't use skill #9, since, seeing as I'm in combat already, and didn't metagame that I was going to be in combat, I don't have that skill that might turn the tide faster equipped. Is it possible that I will still win? Sure. Is it possible that some of my companions will be ineffective? It could be, however unlikely it may seem, it's not impossible, just improbable. However, if they have all learned a skill/spell/talent that could turn the tide, but are locked out due to UI limitations, this is artificial difficulty, since I have the abilities to make it easier, but the game has locked me out of them because they're not on the hot bar.
It's only metagaming if you reload once you see what happens. Otherwise it's just normal play.
So having the Prima Strategy Guide sitting on the desk mapping out what to do for each and every encounter/puzzle/maze isn't metagaming?