I am a teacher, I personally wouldnt bring soft drink to a class party, but I wouldnt be outraged about it. I dont think my kids would drink fizzy drink anyway, but if they did, a small cup wouldnt hurt them - its no worse than juice, IMO, which heaps of kids drink daily. If she is having a reward for finishing homework on time, I would hazard a guess that she is having a difficult class with 'motivational' issues, &is trying her best to motivate them to do their work. Yeah, it should be 'normal', but some kids/ families struggle with things like homework. So good on her for doing the best she can for the class.
I taught 5yo, & I never had a class party with shared foods, etc. Mostly because of food allergies, & it just didnt work so well with the age group. For an end of term treat, I might buy a pack of ice blocks and take the kids outside for iceblocks &a play. We only allowed the kids to bring water to school,so I can see that its a bit contradictory, but I think kids can know the difference betwren a party and everyday rules.