That's BS! I would fight that and force them to install only a new OEM bumper
If that doesn't work find out from the body shop how much they are paying for the reconditioned bumper and then pay the difference for a new bumper. I think a new pre-primed OEM bumper is < $500 from Tischer (getbmwparts.com) and it should only cost $200-300 to paint so total for a new painted bumper < $1k.