RIM claims that Kik "breached contractual obligations," but doesn't get into specifics. Their complaint might be a result of the Kik app retrieving email addresses from devices, and then suggesting them as Kik contacts. Another potential reason, however, is that Kik's messaging service presents a competitive threat to RIM's BBM.
Many BlackBerry users are considering switching to other operating systems, particularly Android. One of the things keeping them with RIM is their attachment to BlackBerry Messenger, which provides near-instantaneous messaging without dipping into their text message allowance. If there were a viable alternative, like Kik, BlackBerry users might be even more likely to jump ship.
Kik's developers are shocked by the app's removal: "We would be surprised and disappointed if there is any truth to this, as RIM has always championed the BlackBerry ecosystem as an open platform." RIM has even shown support for Kik in the past, as they gave them an award at last year's BlackBerry DevCon.
This incident is similar to the way Apple resisted Google Voice. After significant consumer pressure, they eventually relaxed their development standards, and allowed the potentially competitive app into the App Store. Perhaps RIM will experience similarly persuasive backlash.