Voadam
Legend
They are absolutely RAW. And they definitely apply to weapons a character is permitted but have not gained proficiency in.Non-prof penalties are RAW, and listed in the PH. For a MU it's -5 to hit. We always assumed it applied to any weapon the character picked up regardless of class etc., using the logic that says any idiot can pick up any weapon and try to use it but won't know what they're doing.
Whether it allows a character to use a weapon not permitted is not specifically defined other than weapons permitted being a separate thing. Similarly for a thief or magic-user putting on plate mail it is not defined what happens, it is just stated as a prohibition.
Coming from B/X where there is no non-proficiency penalty, just a stated prohibition on certain classes using certain armor and weapons, I came into 1e AD&D and thought the nonproficiency penalty chart meant a 1st level magic-user could gain proficiency in one of the three permitted weapons, and have a -5 penalty on the other two, but they were still not permitted to use polearms.
1e PH 25:
"Furthermore, they [magic-users] can wear no armor and have few weapons they can use, for martial training is so foreign to magic-use as to make the two almost mutually exclusive."
There is nothing physically stopping a magic-user from putting on armor, but it still says they cannot wear armor.