I don’t live in NY but as I understand it, they had to offer this service to every qualified individual. They most likely didn’t have the option to only support certain or just existing customers.

Think of it this way: Had ATT the option to exclude, they would have and abuse it as much as possible. They can’t, so either they follow the law or take their business elsewhere. Leaving paves the path for another company or cooperative who does want to follow the new laws, rather than having ATT undermine at every opportunity. It hurts in the short term, but in the long term it helps. NY isn’t the first place to chase big telcomm out.