I called Reacro months ago and asked why. Had a long conversation. I basically asked what horrible thing would happen to my child after 52 lbs. tethered. The answer was nothing! That is what it was tested to so they can't claim it'll work after that. Maybe it would fail, etc.
As a parent, I'll choose to use it. My kids are not 52 lbs. yet though.
The wording of MUST REMOVE was odd. I don't see how it's different from seats that say to use the tether up to 40 lbs, except Recaro was pretty explicit about removal.
But, I don't use seats untethered.
Of course, perhaps others would be more comfortable removing it. This is just what I decided to do with the information from Recaro.