To your point - yes, adding on an extra 18% doesn't make much sense when they could have simply incorporated that into their menu prices. You will never be certain how the money is divvied up, regardless of whether you are paying additional tips or not...

That being said, patrons are accustomed to paying an additional fee when dining out; so rather than simply raise prices (which I'm sure would elicit grumbling and/or loss of patronage), they maintained the expectation of paying an additional amount for the service they've received.