I just got back from FRA, and did a little detective work.

First, I think kevinsac is misremembering his experience. When he first reported this, he said that it was the contracted ticket agents who said that to him, not the RCC staff.

I went to C2, and there was a LH agent there putting people on the bus. She told me that the bus was only for Lufthansa operated flights - passengers connecting on UA flights leaving from the B concourse had to go the long way. FWIW, there was a sign there that listed eligible flights, and they were all LH operated.

So it seems to me that there are two possibilities here - either this is a LH-only service, in which case maybe it's not reasonable to expect UA staff to know about it, or that UA passengers are allowed to use this, but the LH staff are equally clueless about this.