I used to work at an
AMC movie theatre and they have a similar
policy in place. In fact, it's called the
Guest First policy. While I don't know about fast food restuarants and the like, the movie theatre is located on
private property. Thus, they as a company have the right to bar people from the premises. I don't recall if anyone
was banned, but it was a decision that could be made. Because of these two facts, it could be said that AMC is
inviting people to watch movies and spend their money on over-priced concessions. When you invite someone to your place of residence, they become your guest. As such, when AMC invites you to see a movie (via advertising) at one of their many locations, you become their guest.
Of course, that is if you agree to be called a guest.
As
ClockworkGrue stated, the word guest implies some sort of familiarity, which is precisely why corporations such as AMC and
ClockworkGrue's bagel place want their employees to use the word. They want you to think that they are friendly so that you will be more inclined to
spend your money there. That is it. All it really does is make customers and employees alike feel
awkward (which may actually lead to customers spending
less money). In fact, I rarely called anyone a
guest during my
term of employment. If caught using the dreaded
c-word, I would be told "they're guests," but that was it. Never was it really that heavily enforced of a policy. But that might be because both I and everyone above me were from
before.