The problem always comes down to bad actors.

If I, as a vendor, could have my people do the work of moving and set up and elect not to use the convention center staff, then there's a way to circumvent the convention center staff and fire them all.

You just hire a "vendor" with their own crew to do all the stuff. With no actual work to do, it's justifiable to fire the convention center staff.

If people weren't bastards, we wouldn't need such weird rules. But then again, if people weren't bastards, we wouldn't need unions either.