I'm from the UK and it was so nice to be able to send e-mails and even share images and live updates from Walt Disney World to my Facebook and Twitter accounts.
And this is exactly why it will be free. Not because of MyMagic+. Because of the people that will share on social media sites in real time and get their friends thinking, "Hey, maybe I should take a trip to a Disney park, too!". Free wifi equals FREE advertising.