It’s not that we’re assuming the money is being embezzled by the owner, it’s that we don’t exactly trust that this 18%–which instead of going directly to the server is going to the establishment–will be used to solely fund wage top-ups.
It’s a very safe assumption that anybody who sees an 18% gratuity automatically added to the bill will not tip any further. Some will, but most will be done tipping at that point.