It is like the tragedy of the commons. Even if AWS decided they will not serve oil companies, those companies will simply take their business elsewhere, e.g to Microsoft, or even worse, to Oracle. This will simply have the effect of hurting the company caring about the climate, and benefitting the ones who don't give a damn. Shared resources like the environment can't be protected by the good will of anyone company. They need to be protected by the letter of the law.