Physically speaking, it makes perfect sense that a monster can disarm anything that you wield in your hand. If the monster knows how to disarm you when you have a sword and you're bashing at it with a book, why wouldn't it make you drop the book as well? I think it's fine that monsters can disarm non-weapons.
About disarming cursed weapons, it depends... if curses in ADOM are physical (weapon physically sticks to hand), they shouldn't be disarmable. If they are more psychological/mental (you would really like to leave that sword, but for some reason you can't bring yourself to do it, it's as if your arm had a mind of its own and had resolved to firmly grip the sword) then they should be disarmable.
Both options are slightly exploitable: cursed weapons being disarmable lets you ID weapons as other people have pointed out (and as in ADOM I), cursed weapons being non-disarmable would give you a defense against disarming monsters (just equip a cursed weapon, as long as you have means of uncursing which are probably going to be quite common in the late game). Anyway, none of those two clever tricks are game-breaking at all. So I think any of both options is fine.