I know that some American tourists actually do this, on giving servers cash tips as if they’re underpaid by their employer (since they’re used to it in the USA along with growing up with that mindset) but overlook the fact that servers are paid the minimum wage in the EU, as it’s not “typically” the norm. This practice is being exported it seems, in another region where that isn’t common.
I’ve heard that in some American restaurants, the employer withholds the server’s entire tips (aka. owner robbing them of their money just to put in their own pockets) meaning the server gets nothing. That shit is illegal in the EU if one tries to pull that off.
It is illegal but common practice.