You answered your own question -- to the customer, it's annoying.
... and to the restaurant owner, it discourages walk-in traffic.
It's immoral in that it's dishonest (We have a word for reserving space at the restaurant by claiming to be a party of two, but not being one -- it's called lying). If you wanted to do this in the moral clear, you need to speak to the restaurant owner first, and see if they'll pre-allocate you tables.
It's scalping. It raises the cost of a dinner in the restaurant without additional gains for them. Obviously restaurants would not go this.
It raises the monetary costs of dinner for people, who want to pay to avoid waiting. For the normal-waiting patrons the strategy feels like increased congestion. (And they bear the costs.)
Why do restaurants have queues anyway? Why don't they just raise their prizes until supply meets demand? There's more than one scholarly article about this question.
Because you're fucking up the social contract that restaurants have with their customers.
It's a valid point how the artificial constraints are the envelope and the time, because the way she listed the rules was, clock starts when you open the envelope - it's almost a a ploy, or a method of trying to distract someone. Imagine, if you were handed an envelope and were told "there's money inside there", wouldn't you immediately start to wonder "how much?" and you would immediately want to open it. She restricted this thought by applying a 2 hour time constraint, from the time you open the envelope, which almost forced the students to think twice about jumping to the money. Instead, all the groups achieved something without ever really having to open the envelope. I think it was interesting, and a job well done.