IIRC it's because it requires an engineer to check the signal levels, I think because it's using different frequencies to the normal DOCSIS 1 modems virgin supplied, and it's using a wider range of frequencies it needs an engineer to make sure that everything is ok.
I suspect it also slows down the rate of adoption, giving them a chance to spot possible problems in each area before too many people will be affected.
Look at the modems status - it's usually http://192.168.100.1 and it's normally root/root to login. If it's anything else it *usually* says the details on the first page.
As to what is "normal" for 50Mb. . . . no idea.