Have you ever heard of bending an Upper receiver?

[rasp65]

The barrel nut is being torqued to the receiver not the barrel. As mentioned by Ullr the receiver is only held by the tiny indexing pin that locks the barrel extension to the barrel. The threads the barrel nut attaches to are on the upper receiver so that will be subject to torque. Clamping with a barrel vice still puts torque on the upper but it is not held by anything so it can rotate freely and avoid damage.