They're welcome to send that IMO. And sites are welcome to try to detect and ban agents (formerly: "bots").
As long as it's not wrong/immoral/illegal for me to access your site with any method/browser/reader/agent, and do what I want with your response. Then I think it's okay to send a response like "screw you, humans only"
Paywalls suck, but the suck doesn't come from the NYT exercising their freedom to send whatever response they choose.
Yes, that's what I mean. Attempting to tell people not to do something is like setting a robots.txt entry. Only a robot that agrees will play along. Therefore, all things have to be enforced server-side if they want enforcement.
Paywalls are a natural consequence of this and I don't think they suck, but that's a subjective opinion. Maybe one day we will have a pay-on-demand structure, like flattr reborn.