It does come down to personal responsibility. I know for myself I'd opt to bring my own diet soda.
Also this. Between many food allergies and dietary/health restrictions, I've learned not to expect other people to have the food choices available I need. You know the old saying, "Prepare the child for the road, not the road for the child." Try to think this way when it comes to food now. There will always be food situations that will not be ideal.
I bring my own nuts, string cheeses and proteins I need, in case there is nothing available I can eat. People sitting with me will often remark, "I wish I had brought my own hummus," when they see me eating mine.
I usually drink water. With so many people carrying around their own water bottles nowadays, you can just bring your own filled with water and a lemon slice or sugar free Crystal Light packet.
Or bring your own diet soda. You can't even depend on them picking your flavor, unless you specify, "Please provide ____." For me, I don't drink caffeine later in the day. Most places that have a diet drink available, it's usually Diet Coke/Pepsi, with caffeine. Or some lethal tasting sugar-free punch.
You could also have requested your specific diet soda be made available. But, then you show up late to the lunch, and the 6 cans they had were snapped up by other people ahead of you. I've often found, when they put it off to the side for me. When I get there, the person who put it off to the side is off doing something else and no one else knows where it is. And I have to wait 10 minutes till he comes back, or for them to find the item.
Life is simpler to just bring my own specialized stuff I need. And I will always know there is something I can eat, since I am bringing it.