It's a bit of a grey area.

Coffee is full of lipids, which build up in the sewer just like pouring oil down the drain. Coffee shops need a grease trap for that reason. (Google says UK does this, I know it's legislation here in Aus, we mostly copy your homework anyway)

One small bit of a cup isn't going to break anything, but infrastructure planning and legislation has to assume that a percentage of people will do it, some of them all the time.

I don't think she should have been fined and I do think that 3 men ganging up on a small woman to berate her about a coffee is bonkers.