Is TK like pre-K? If so, yes, I'd be annoyed. That's too young for the parents not to at least be forewarned. If she hadn't told you about it, she could have chugged along daily and racked up quite a bill / used her months' worth of lunch account money in a week or whatever. Two dollars a day has no meaning to a 5 year old, she could have easily chewed up $20 in a couple weeks, $20 of sugary goodness for a 5 year old is a lot!
In our school, the little grades aren't allowed a la carte stuff. In early middle school, they are allowed for certain things only (like extra entree, I think, I can't exactly remember but it was limited in some way), it can be turned off if it is a problem. After that, its between parent and kid, I believe.
My kids were buying waters in middle school, which I didn't realize until they ran their entire lunch account dry. I was ticked (at them) because their schools have those fancy water refilling stations and we have dozens upon dozens of reusable water bottles. It wasn't on my radar that they were buying WATER, but it should have been I guess!
My son buys breakfast sometimes, and even though he just ate breakfast at home, I'm (generally) ok with that. I figure it is just a friend / killing time thing.