A restaurant who takes reservations is entering into an agreement: you promise that you will eat a meal there at a certain time, and they promise that they will have a table ready for you at that time.
If the restaurant is all booked up, they turn people away. If some of those reservations are held by scalpers, and the scalpers don't find a buyer, the table will be empty when it may have been used if not for the actions of the scalpers.
What the students did was to break this unenforceable contract, and convert an agreement to make a transaction at some point in the future into a pure option on their part. That's stealing value.
If the restaurants want to do this, that's one thing. If the restaurant attempts to cultivate an image of being the hip spot to be seen, then it's in their interest to seat anyone dressed nice, or has money to throw around, or whatever, preferentially. But I would expect that they take this factor into account when setting the wages of the maitre d, just as they account for tips when setting the waiters' wages. Thus, the restaurant captures the value.
If this became prevalent, I would expected restaurants to move back to a model of calling names, or asking the name as if they can't tell from what number buzzer you had. Psychologically, buying a buzzer would seem okay to a lot of people who would not be comfortable if what you were selling is "listen for Charlie, party of 4, and say you're Charlie".
All the above applies equally to buzzers or waiting in an physical queue. If the queue is visibly large or if the host says it's a 45 minute wait rather than a 20 minute wait, some patrons will go elsewhere.