1. While they aren't currently accepting new API partners, that hasn't always been the case. There are quite a number of apps on both the iOS and Android platforms that are using the API, and some are very highly-regarded and widely preferred over the official apps.
2. Groundspeak is under no obligation to follow the "open" movement. If they feel it's in their best interest to keep the API proprietary and closed to new partners, I don't see why they shouldn't be able to.
As for the rest of your post, I wholeheartedly agree. I think there must have been some technical or legal reason that forced Groundspeak to prematurely kill off the old app and roll out this unfinished one, and they aren't at liberty to tell us the reason why.
While I think you're irrationally forgiving with them as to "why," I agree there's no law requiring Groundspeak to be anything other that closed, proprietary, and anti-competetive.
At this they excel.