If Amazon employees started tipping 0% at restaurants in Seattle, they would be legally correct. But that would certainly impact the way restaurants needed to operate.

Sometimes communities get built up on a shared system that involves a certain amount of shame, embarrassment and feelings of communal good and following norms in order to continue.

Open source software seems to be that type of community. Sure, you can be a freerider and tip 0%, but at a certain point others in the community may glare at you with a "really?" face and of course restaurants will eventually just raise their prices.