Hello John,

it is difficult to say I suppose that shutting down the link is part of their procedure

on PE side they just need to apply a service-policy inbound to accept your marking and to drop and/or marking down non conforming traffic.

On cisco equipment this doesn't need to tear down the link, and likely it is not needed on other vendors' equipment

For this reason my guess is that it is a part of a procedure they follow not a real technical need.

Or they prefer to do even a small change under the umbrella of a declared maintanance window to be on the safe side.

Hope to help

Giuseppe