I'm sorry, a meeting at work (or parents evening) is very different to general supervision/work time.
When I worked in an office I would definitely eat snacks at my desk, but not in a meeting. I am working at both, but different circumstances.
Likewise, DH would not eat in the classroom whilst teaching, but definitely would during recess or a games lesson, because he usually had to skip lunch to sort out x's exam paper or Y's parents complaint re: nothing.
I certainly think in after school club the TA is in a more relaxed period of work and can't be compared to parents eve/meeting.
Also, the kids presumably have the option to go in and out shade etc... Staff are often posted at certain points so that they have a good view of the entire playground, and she doesn't have the opportunity to go inside and sit down if she is too hot.
I told DH about this thread and he's asked me to bring 3 ice lollies to school at recess tomorrow