The simple answer should be in the menu and outside the restaurant that the tip/wage is "obligatory" but the official answer is its non obligatory . Why not just list the tip as servers cost on the bill .
If given a choice many tourists (visiting a restaurant for once only) would not pay any tip let alone 8% as you suggest, the majority of the worlds country's don't have the same tipping culture as the US and therefore are not expecting this misuse of the term tip . As you say regular visitors have no excuse not understanding the custom, but there are always first time visitors, I don't think this information is given out at immigration .
Its rather absurd to suggest not being aware of a fact is hiding ! just maybe those that "don't know any better" just don't know any better ! Why wait for those who as you say "doesn't know any better" to simply ask, when it could be made clear to all . Without better information "cheap foreigners" will continue to be " cheap foreigners " sadly .