Can anyone try installing it to a USB stick to see if that's any different?
I find it hard to believe its this way by accident, since AFAIK the game doesn't know it's on a HDD vs DVD. Typically Bethesda games have utilized a HDD if present to cache resources to speed loads, even while running off disc.
Either A) this is some really bizarre bug that no one noticed.
Or B) having to constantly seek between the cache partition and the game data caused such performance problems that they decided this was the best way to solve it.
Which is weird, because if they were able to detect its installed on a HDD and then lower textures, the more obvious solution is to disable the caching instead.
So it's just weird. It requires some sort of explanation or acknowledgement on their part, even if it's just a "whoops, didn't notice that bug!". Have they acknowledged the issue anywhere?