We may not agree (and I don't agree with Apple's move here) but it's hardly surprising that they would shut it down for their direct competitor platform.
Let's not forget that it is/was a paid subscription on Android, so offering it for free on iOS and keeping paid on Android would still give them a significant edge. Somebody else in this thread mentioned that Shazam is still available on Android even after the acquisition, so it's definitely not as obvious as you suggest.
Not only that, but presumably if Dark Sky becomes the base for the iOS default Weather app, then there won't be a DS standalone iOS app. So they wouldn't just be maintaining a port, it would be an entire Android app with no iOS equivalent.