The school behaviour policy probably won't talk about sweets as a reward, but I've been advised in schools previously to use sweets as a reward, and sometimes it can work really well with a tricky class to get them participating and turn a corner.
I teach older students, so I suppose it is a bit different, as most of them can buy what they like in the shop on the way home from school anyway!
But I genuinely don't see it as a massive issue. Especially when there are lots of primary schools in England who don't seem to deal effectively with very serious behaviour incidents.
I don't think it's ideal on a daily basis, but there's presumably a reason she's chosen to do this half way through the year, and it's probably not because the class were behaving perfectly before.
OP- you don't "have" to talk to the teacher, and honestly I'd seriously consider not doing so. It's unlikely to change what's going on, and if you do make a huge issue of it, she's likely to just look for a job elsewhere. Most teachers are in a position to be able to pick and choose now.
For the PP who said why can't she use 30 minutes of games at the end of the day as a reward- this wouldn't be allowed in most primary schools because of lost learning time. The primary curriculum is absolutely rammed.