alpha device will receive an update "in the summer" including BBM.
Sure, we could not believe it won't be on BB10. But it was supposed to be one of the biggest problem for RIM (QNX / Playbook times) to implement it.
So the good news is both the timing and the fact that they DID it !
Clear enough ?
If it's coming to BB10 (as I think we all assumed it would), it makes sense that it would surface on the Alpha devices first. This way BBM-connected apps can give BBM a workout and RIM can receive good feedback on any problems/concerns.
I don't think that Playbook Beta releases are the place to allow initial testing of an app as significan as BBM since people tend to treat the Beta releases as final. Since the road for the Playbook eventually ends up at BB10 anyway, BBM will most certainly be available for the Playbook. But I think it would be a disaster if any major flaw surfaced in a publicly distributed Beta release before a developer Alpha.