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It doesn't sound grammatical to me, but I think they're trying to hard to sound "polite" by avoiding a command (imperative) form altogether.
There are some such "over-polite" expressions that sound icky to many Koreans yet persist among Korean service providers. The most notorious example is putting "-시" to every subject. For example: 커피 나오셨습니다, or 이 스테이크는 고기가 얇으세요 (yes, I really heard that once).
As for why such forms persist, the explanation I heard is that most reasonable Koreans consider these forms wrong, but some unreasonable grumpy Korean customers with neither proper education nor basic decency thinks that they are the king (literally, "손님은 왕이다" used to be a common phrase in many shops), and that every sentence that's addressed to them must end with "-십니다" or something similar.
(We have a word for those kind of customers: 진상.)
...and guess which kind of customers would make a ruckus (and make the service worker's day miserable) when they don't like the expression they heard?
This news article describes the same story: "치즈 올라가세요" 손님 왕대접에 망가지는 우리말
More news articles I found regarding these expressions:
