Planting a gun on a Terrorist or throwing the gun in the path of a Terrorist is.. Well, gunplanting (just to clarify though I'm sure we're all well aware). Regardless of how much ammo it has, the player is still technically "planting" a gun on a Terrorist. Only reason the rule is in place is to prevent any accidental deaths like Chrono mentioned since scenarios like the following can occur rather easily (assuming they have auto-switch on):
>T is facing CTs
>CT decides to throw empty gun onto the T
>T [automatically] pulls out the gun while facing CTs
>T is killed for aiming a gun at the CTs
Don't mean to call out whoever it was who told you otherwise, but I don't think it should be permitted under any circumstances. Though this only applies to moments when it's intentional. Even though the CT(s) may be slain, it can really put people off [ especially when they're close to getting LR] if it becomes a common occurrence.
Pertaining to the thread, if multiple warnings were given on top of multiple occasions in which the gunplanting/baiting occurred then I think the CT ban was justified. As previously mentioned, typing 'kill' in console is a far better choice as opposed to baiting which may likely result in the death of a T as Mega said.