How would a company enforce that? Is the NYT going to start suing customers? Cutting off their access?
[EDIT] I'm guessing they put that "No Tablet" option to ensure they get a licensing chunk from vendors who might want to sell inexpensive tablet that run on the AIR platform.
That, and now that people are paying for subscriptions they (pragmatically, if not strictly legally) have to guarantee a particular level of user experience.
Even though AIR is cross platform (write-once look almost same almost anywhere) having only a single platform that's legal to use helps cut down on customer support headaches.