I'd see it more as an issue of
accessibility.
You can't always hop off the aircraft to buy your lunch/dinner (or whatever food) even if the aircraft does get on the ground, if you are on a tight turnaround, whereas a lot of ground-based people can physically get to a shop to procure their own lunch.
Because they do not have an easy access to food (they can't even ask someone else "in the office" to get it for them
), I do think the employer would be responsible for providing the flight crew with an opportunity to have the food (even if the staff had to buy it, if the company is being a really tight whatever).
I do believe that not eating properly can be a flight hazard, for the same reason already stated by RTA.
I do not really see that hyginie should be a major issue though, whether the crew prepared their own food at home or bought it from a kiosk or the food is provided on the aircraft. It's just called basic food hygiene that everyone should observe, whether one is preparing food for him/herself or others.