In Belgium it is common knowledge that cash-only places don't report everything to the feds.

For the same reason those food establishments had to get a government approved White Cash Register, because there was so much money disappearing.

If you don't do the accounting, it appears that the fees on a credit card are greater than the cost of cash and so restaurants that haven't done their accounting closely often think they're getting themselves something by not accepting credit cards.

The cost of a credit card is very obvious. The cost of cash is many small things that are very hard to see and thus very hard to account for correctly.

Small cash-only businesses sometimes have other motivations to stay that way, like: The books can say whatever the owner wants them to say.

If you only accept cash, you can report whatever you feel like as income* and avoid paying taxes on the rest.

* As long as you don’t raise any red flags in your accounting, which can be difficult.