I'm a graduate taking chemistry for the first time, and my RVC offer stipulates I need an A grade:/
The RVC and Nottingham in particular seem to be getting tougher about it, but I think it's largely an effort to be fair.
A-levels and Scottish Highers are standardized and allow you to compare easily between students; university degrees vary wildly in their comparative standards. The majority of students will get a 2.1, but the way a degree is classified at two different universities won't necessarily represent the same standard. There are 325 institutions listed by UCAS as able to confer degree status, it would be unrealistic to expect academics to know the relative merits of each so that they could compare between two students.
If they use A-levels, then it's easy to tell between people at a glance. Also, you can resit A-levels and they are relatively easy to access for learners outside a traditional academic institution; whereas a degree isn't.
I think it's pretty fair; cutting grads a bit of slack where they have proved they can perform at a higher level, but not cutting them so much slack that it disadvantages sixth form leavers is a hard balancing act.