I too had a similar experience on a Delta flight from PBI to LGA but my two people were a man and a woman who were in their 90's and had trouble
getting down the aisle. Before the flight took off I did notify the FA of this bad choice to sell the emergency seats to people that could not obviously help in an emergency. Victoria in a very "snippy" voice said they could not discriminate!
When I got home I notified Delta and they did not even answer.
It was not a full plane and those "oldies" could have sat somewhere else.
I think those seats are more expensive and all the airlines care about is money and not the safety of their passengers.