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Litigation can be one reason, but I think the more likely reason is that people want to avoid confrontation.

Could you tell the person taking the pizza that that is inappropriate behavior? Sure. But that is confrontational. The people who might set the boundary are worried both about how they will appear to others (am I being a bully?) AND about the possible repurcissions (is the guy I'm telling off going to yell at me or threaten me?)

Its far easier to just stop buying the pizza.