Our council laws say no "(xi) sewage, manure, faeces or urine;" So I'm guessing nappies with poo fall under this bracket. Every waste treatment centre will have different rules and it depends on where your rubbish goes to as to what the law is.
We have single bin recycling where everything goes in, then gets undone from the plastic bags, tumbled, sorted, magnets pull out the metal etc. I wouldn't want to work there, for sure!
I've never heard of anyone being charged for putting nappies in the bin. I believe they have cameras inside the trucks in case they need to go back and find where something came from.