| I was in the restaurant business for over 30 years. Most (not all) servers are young and think the reason most people order diet drinks is because of weight. It is management/owner who should train their employees about health problems and as to why customers order certain items. Mistakes are made but most of the time it is because the employee is in a "hurry".
The drink syrup at one time came in a metal tank and the hose connection for diet and regular drinks could not be interchanged they simply did not fit. Most syrup comes in a box today and I am not sure how the connections work. (Been retired for a few years)
My point is it is usually management's fault for improper training not the server.
Complain to the manager not the employee if you want action.
Another problem I have is caffeine. I have a heart problem and must watch the caffeine and I can not tell by taste. | 
