It is an exploit to use/sell/trade/interact in any way with an item that was acquired from a shapeshifted creature. Tenser's sword, drow blades ect.

If you see one on the ground or disarm a shapeshifted opponent using one do NOT pick it up.

hmm...why's that?

I believe there were a few reasons for this. It's a very old rule from our Urath days. These items don't conform to our item guidelines. This includes certain information (ie the blureprint name, tag name etc) that we use in our scripts to govern the behavior of equipment (such as being craftable or breakable).

Another problem is that such weapons are essentially free and created on the fly at any time. If you know how to separate them from a shifted character or NPC without them disappearing then you could essentially create/aquire any number of these weapons out of thin air. Which is a bad thing.

The wording is slightly ambigous and so I'll clarify that this doesn't apply to a shifted character using the items normally while shifted. Finding ways of keeping these weapons however is also a bug exploit since normal game mechanics are designed for the weapon to disappear when a polymorph ends.