Best guess is while it probably provides some tactile stimulation, it doesn't provide enough to get the job done, so to speak. If he wants to get off, it has to be from the aneros and their attentions to him.
I would assume it's a restriction for erection. As in, normally as he gets hard (and it wants to point "up") he could just shift around a little bit and "roll it around" to a comfortable position. The tube means he'd have to bring his hips basically all the way up to do so, which the girls probably won't allow.
