When you put it that way - yes it is odd.

From a general business point of view, they are promoting a page and social media seems to be the way to go about this. How best to promote their own page but through the employees, who tell their friends who tell their friends etc etc? They have to start somewhere.

In your case, I think it is odd that they would make a big deal of banning FB etc then ask you to "like" them. lol! I certainly wouldn't feel obligated to "like" their page, given their attitude & lack of trust in their employees.
This is what I mean, I don't care about 'Like'-ing the page, whatever, I know they can't see news feed etc. It's just the fact that they're SO strict about using social media at work. Double standards really.