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.