You're looking at it wrong. They didn't cripple it to sell more phones, they developped the product when they still had good market share in the mobile world and wanted to sell PlayBooks into existing BlackBerry accounts. It was a way to bypass IT having to manage another device and piggyback on the already-approved BlackBerries in enterprise (think FIPS approval, etc). They did not expect then that nobody would want BlackBerry devices anymore, and that it would be a major handicap to the PlayBook to not be able to function as a standalone device.
Short-sighted yes, malicious tactic I think not.