I wouldn't say Americans fiercely defend the tips system so much as don't think about it all that much -- it's just the way it's been for most of our lives (just like driving on the right-hand side of the road and predominantly using dollar bills instead of coins). Of course, there's no law prohibiting someone from starting up a "socially responsible" restaurant that promises to pay servers a living wage and full benefits in exchange for disallowing tips (fast food sort of does this, except for the living wage and benefits thing), but the current system has a lot of inertia.