I Think the real reason they use a conveyer belt is to
make sure *that* the visitors don't
get too good of a look at the crown jewels. There is no better
way to deflate the hype
you've invested than to let people stop and stare.  Keeping the
people moving insures
that they at least won't get bored, they probably won't notice
the dust in the case, and
they *hopefully* will leave wanting to see it again (for an
additional 25 cents, of
course!).
     It's kind of like those soda cans that prevent you from
sipping that last little bit.  Or
maybe it's more like a XXX peep show?!