The news doesn't really come as much of a surprise, although BlackBerry had gone on the record saying they wanted to "focus exclusively on smartphones for the time being," before moving to tablets. The demand for an iPad/iPod Touch-enabled BBM was pretty huge, so there's no doubt that played into the decision to give in as well.
The main reason BBM wasn't available right off the bat for WiFi-only iPads/iPod Touch is that BlackBerry doesn't like BBM being on a device that's not constantly connected to the Internet, a device that's not always on. That's also the reason there's no "log out" button on BBM either. BlackBerry's philosophy is, if you have BBM installed, a message sent to you should always be delivered. In the case of WiFi-only devices, that's not always the case.
No doubt this move will help the userbase grow. Just by how much, remains to be seen.