Alton Towers is currently installing many devices across the park with the aim to establish resort wide WiFi coverage.
Following a smaller scale installation in the hotels which offers guests free access it is expected that any extensions to this network will be free of charge as well.
The most likely explanation for this installation is the recent interest in offers through social networking websites. Companies are very keen to 'push' offers through to customers mobile devices to entice further spending. Due to the resorts location this is not currently possible with the unreliable mobile network coverage so a WiFi network would provide a solid backbone through which to provide such a service.
Whilst a launch date is currently unknown the devices have been testing periodically so it's worth checking for WiFi connections when you next visit the park.