Here's an example.
Fighter Fred is shooting an arrow at Barbarian Billy's (lol) greataxe. Fighter Fred strikes the greataxe with his arrow. Now do you really think that puny arrow, with its miniscule mass, is going to wrench the axe, which is several pounds heavy, out of the barbarian's grip? Consider that normally, Fighter Fred would have some difficulty even trying to knock that axe out of Barbarian Billy's grip if Fred was wielding a halberd. Comparatively speaking, that arrow's force of impact is pathetic. It may penetrate a thin layer of armor, but it doesn't carry the same knockback factor as a warhammer or something. At best the arrow will lodge in the axe's haft or, perhaps, dent the thick axehead, neither of which is likely to carry the force necessary to wrench it out of the opponent's grasp. Unless the opponent is completely unaware and not really expecting a fight (in which case, why the hell is he even holding a weapon idly, rather than leaving it sheathed?).
Now, I'm not saying it's impossible that an arrow might be able to disarm a melee weapon, but it seems rather unlikely (then again, I'm not exactly sure what the specific force of an arrow is compared to that of a halberd or something). I'd be more inclined to believe that a throwing hammer or bola might carry the necessary raw inertial power.
Likely, the feat from CW (which is based on a feat from Dragon Magazine's issue some time ago that had an article on Robin Hood) is limited to 30 feet because a ranged weapon loses some of its momentum and force after a short distance, and because it's hard to aim for the right spot on an opponent's weapon (which is comparatively much thinner than his/her body) from any significant distance.