So one should just start fighting and hope others follow? Why not just be a hypocrite, we humans are good at it, that way you can promote the social norm you wish to inspire with a much smaller cost!

If you're in a population of similar agents, you should expect other people to be doing the same thing, and you'll be a lot more likely to lose the war than if you actually fight. And if you're not in a population where you can rely on other people choosing similarly, you want a policy that will effectively force everyone to fight. Any action that "promotes the social norm" but does not really enforce the behavior may be good signaling within the community, but will be useless with respect to not getting killed by barbarians.

Why bother to learn what global warming if it suffices for you to know it is a buzzword that makes the hybrid car you are going to buy more trendy than your neighbours pick up truck or your old Toyota (while ignoring the fact that a car has already left most of its carbon footprint by the time its rolled off the assembly line and delivered to you)?

A person who only believes in the signalling value of green technologies (hybrids are trendy) does not want a social policy mandating green behavior (the behaviors would lose their signaling value.)