I wonder what the tower and airport authorities would have done if the pilot declared a formal emergency that was a threat to the passengers? Say someone could not stand it anymore and was smoking in the lavatory, and threw a lit cigarette but in the towel bin, causing a smokey fire?

I assume the airport would have found a way to get the passengers out and to safety.

Now, why couldn't they simply declare the same emergency and get everyone out? I fear the that "the rules" simply prohibited it.

My trapped on a plane experience: Mid-70's trapped on a 747 grounded overnight by a snow storm at Dulles International Airport, which had been diverted from NYC when JFK shut down due to same storm; we were coming in from Frankfurt. No food, no water. Dulles was my destination. We sat there, flew back to JFK the next AM, went through customs, got on another plane and flew back to Dulles. Stated reason by Pan Am: customs procedures.