Historical accounts by conquering Roman officers describe Iberian dancing girls, who sank, with quivering thighs, to the ground. Orientalist paintings and lithographs exist with dancers on their knees, some with a tray or sword balanced on their heads, some without. There are descriptions of movements, done in a kneeling position, in the course of an Oriental dance, in several eye-witness accounts from Egypt, Syria, Turkey, Central Asia, Azerbaijan, etc. King Farouk, the last of the Ottoman rulers of Egypt, was notorious for many things, among them his love of Oriental dance and dancers. Now you know why the net covering with its inconveniently placed zipper.
Some famous dancers can occasionally get away with doing it now.
If somebody is taking a photo, the dancer must stand stock still. That is why some dancers use a part of their dance to go from table to table and pose for photos with the customers, or why they stop and pose if somebody comes up onstage for a photo. I have no idea what the reasoning behind that was. During that dance, Negwa did floorwork, lying on her side. OK, then explain how she got away with more floorwork, in another dance later in that show . . .