I'm not sure they like it, but it is what a lot of people keep them in. People also insist that you have to feed them beef-heart and say that there aren't any <"plants where they live">, and I'm not sure they are right either.
The discus display I had, that they regularly bred in, was half wooden roots and half lillies with surface pads. This was a tannin rich set up and they spent most of the time in the planted area.
I did get moaned at by some dedicated discus fanatics but I figured they were happy as they were breeding and out and about in a shop setting.
I think it's more a case of discus keepers prefer a minimalistic setup because they are easy to keep in them. The boldest most settled (which is easy to tell with discus colouration) have been in tanks with lots of cover.