[fredkbloggs]

Many owners of bars (and similar cash businesses) skim some part of the business's cash for themselves. This has two effects: first, it provides them personally with undocumented and tax-free income; second, it greatly reduces the business's profits (because you're not just keeping the profit, you're also keeping the cost), in turn reducing or even eliminating its own tax burden. As far as I know, this is illegal in every jurisdiction that has an income tax, but it's still a very common practice. If you were the owner, rather than an employee, of such a bar, that would make this an attractive proposition, even ignoring the cost of the card processing.

As an employee, it might still be attractive if you're in the habit of stealing from the owner. Obviously if there's any documentation that the drink was served, it won't work as the contents of the till will be wrong and usually that comes straight out of your pay. That in turn takes us into all the various ways the help can steal from owners, which is another topic altogether.

If one assumes that everyone involved in the operation of the bar is law-abiding, then your point stands. But that's less common than you probably imagine.