I would agree, the council or other has provided the drop toilets, if they say don't empty porta potties in them then why try to find excuses to do it. I'm sure you would like the job of trying to pump them out when they go wrong.
Surely you can plane your trip a little to be able to use dump points, if your porta potti is full it is your problem don't make it someone else's or am I being to critical?
Same with rubbish, if the bins full just don't bag it and leave it on the ground take it with you and I don't even what to start on about toilet paper and s$%t everywhere.
That's my rant for the day.