The grade for one speed test represents something that the user may already recognise as being a problem, and may do something about. The aggregation of grades can highlight ISPs that are afflicted with end-user hardware that could be improved, I suppose. Is it even possible to detect ISPs afflicted with buffer related latency issues within their infrastructure using an environment where people running tests of any kind have huge CPE or Wifi buffers already?
Yep the ideal situation is to have people use their entire link bandwidth and yet any additional stream should be almost as low in latency as idle latency is. That is what a grade of A+ would be highlighting, and many people have got there after seeing a poor grade and doing something about it.
Regarding capping of speeds I'm just pointing out that a "cheap fix" for some people has been to throttle especially the upstream bandwidth (somehow) just below the upstream rate discovered to be the max - which reduces the opportunity to fill a large upload buffer in the modem. It is a kludge but without replacing equipment or re-flashing firmware, sometimes is the only option open to them.