I like to think a CO uses reason when giving a charge like that. If it was adults they should have known better. If it was young kids it might have been treated differently.
When I was a kid in NB my Dad a friend of his who was retiring soon as a CO and my two brothers were out fishing for Browns and Landlock our first time. It was soon going to be dark and in NB you cannot fish after dark. We had no bites all evening, but then my youngest brother finally got a bit a nice eating size landlock salmon, but the minimum size was 14 inches I measured his fish and it was just over just under 13 or around there and I told him he would have to throw it back he didn't look very happy. My Dad's buddy ask my brother who was probably about 8 at the time if it was his first landlock and it was. I still remember the CO saying to US "Eh looks 14 inches to me".
Point is sometimes a law might be broken with good reason but other times people should no the rules. Those were adults and they should have read the regs they deserved to be charged.
There is no Party fishing in Ontario as well there is no Party hunting small game or migratory game.
I have heard people in the past (even on here, but they were corrected very fast) that they party hunt for ducks or grouse etc.