If there's no chance that you might face any form of punishment for not paying, then there is no fare system at all, period. It's not honour system, it's a systemless system. You pay a fare because it's required, not voluntary. POP is the honour system, because you are very seldom checked - all the times you pay and are not checked, to GO/Viva you've paid on your honour. I don't know what literature would refer to any system that has absolute zero chance of punishment for not paying a fare as a valid fare system. That's a donation, not a fare.
The act of having a fare makes it a fare system.
Saying
you have to pay $2 to get on this bus, but we won't stop you if you don't doesn't make it a donation, which is by its nature optional. If anything, it probably just means that the cost of hiring an inspector exceeds any fares that would be recovered by hiring one.
It's like growing up in a rural area where people would set up a stand at the side of the road with vegetables for sale and a box for customers to put their money. People come, take the vegetables they want, pay for it (not optional) by putting their money in the box and taking appropriate change from the box, and drive off, all with no attendant. No, I am not making this up, this does happen.
Example #2. In high school I worked at a Conservation Area where admission to the park was $2. On weekdays, there was no staff members on duty. There was a sign at the entrance stating that there were no staff in the park and the required (not optional) admission needed to be placed in a dropbox at the gate. And every Saturday when I showed up to work I would unlock the dropbox and dump out a couple hundred dollars in loose change.
That's what an honour system is. The only punishment for breaking the rule is the damage to your personal sense of honour (and others sense of your honour if they see you). It's a wonderful thing that these days such systems still exist in some places. But I get the feeling that counting on nothing more than people's decency is such a foreign idea to many people that they simply can't believe it exists anywhere.
It's like on-street parking here in Toronto. You park, pay at the machine or metre, and there's a chance that parking enforcement will come along and make sure you paid. No different than GO trains. Personally, I wouldn't say that on-street parking works on the honour system in Toronto.
Anyway, this is a semantic argument that I shouldn't have started in the first place.