It's left over from Neverwinter Nights, in which it makes the user immune from paralysis and movement speed debuffs. Maybe it does the same thing in KOTOR, but it could very well do nothing.
It's entirely possible to have armor that doesn't restrict Force powers, though. The restriction is through the base item number. If you make a new base item type that is otherwise a copy of armor with the same appearance and such, it won't have the restriction.