"Supervisors are on a far greater wage then junior Staff"
I worked in a coffee shop while at Uni, I also worked in a pub and a mainstrem video shop, all part time. In all three of these jobs I was the lowest grade, the grunt basically. My supervisor was seen to have athority over me, discpline, work rota etc but the differences in pay in all the jobs was very little, maybe £1 - £2 an hour.
If the supervisors do much of the same work as the juniors i.e. serving the public I see no reson why they shouldn't get a share of the tips. The extra pay they recieve is to reflect the extra responsibility they have in their role as supervisor.