Here is a difference between EU countries. In France, you cannot even tip by credit card or anything else that can be controlled by the employer or the state. It's only cash and at your discretion. Thus, it cannot even be a taxable event. However you need to have some cash if you want to tip.
I think you can, you have to ask "I want to tip x euros" before the waiter puts your cc in the machine. It feels really awkward and so most don't do it
Indeed you are right, you can do it! However they need to add a line to the invoice to sell you something more (e.g. a bottle of water). And when they will report what they have sell and what they have buy there will be a difference. So for few customers it may be OK, but they can't do it every time.
