How is any EULA enforced? I think the goal is not so much to "prevent" as it is to clearly indicate that it isn't "approved."

[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.

Edit: On further reading everything I speculated below is pretty much wrong and inapplicable.

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.