My non-Scandinavian girlfriend never understood why people pay for the bus there if they never get checked. Or why people bother to leave money in a little box at an unmanned coffee/waffle/etc stand in the middle of nowhere. But most do as it is the right thing to do, guilt free.
And if there is a slight chance the tax authorities might contact him the guilt/embarrassment is worse than any token fine.
They aren't the same from an ethical perspective.
You understand that you enjoy living in a system that you can trust. You understand that being able to trust the system means that the system has to trust you. You value a working system more than a petty dollar or some other short-sighted piece of self-interest, because you get more benefit from having the system work in your favor than taking a little for yourself but eroding/ruining the system in the same go. If most people do the right thing, the system works and everyone benefits.
Simple as that! It's the opposite of the tragedy of the commons.
In non-utopian countries, you have to make an advance deposit into the system and may never do enough to make it universally trusted. People may never stop jaywalking, or taking a free coffee/waffle/etc. from the unmanned stand, or cheating your taxes with family income and business expenses. But I'm doing well for myself, so what am I really losing by doing the right thing? Life already put me ahead. I'd rather contribute a small part to a working system I can trust, than to grab another small pie just for myself.
This only works if you never piss anyone off, have no enemies and the government only stands to look bad from going after you.
Obviously accepting donations in an illegal manner is a stupidly low hanging fruit for someone who wants to screw you over. Sure you'd probably only have to pay back taxes (or whatever) in the end but it's a massive hassle and better to just keep it on the down low.
...which they probably wouldn't.