>.."distribute on the App Store, without an explicit exception".
That is a good idea, thank you. I'll add such an exception so libzdb can be used by any iOS/OSX open source app on the app store. I'm a big fan of Open Source Projects and happy if they can find some alternative income such as from the app store. Growl for instance seems to do good there which I'm glad to see.
Otherwise, my goal with using GPL is to preserve the concept of copyleft with the library. The GPL only applies if you plan to distribute libzdb to third parties. In that case, my hope and the reason I use GPL is that any modification done to libzdb can be contributed back so others can benefit. Otherwise, you can do whatever you want with libzdb without any restrictions.
If you're really concerned about people not contributing back their changes, I'd first consider whether it'd be due to their unwillingness or maybe a cumbersome process of doing that.
For instance, I can't find anything on the website that would explain how to submit a patch. There's a mailing list and it'd be great by 2002 standards. Hosting this on GitHub (or any other site with a forking workflow that's just as smooth) would probably solve your problem without having to resort to imposing legal restrictions. There still would be a few companies/people unwilling to help but only a considerable minority.
If that is your intent, why not place it under LGPL. That way changes to the library must be published but the program that uses it can remain closed-source? As you have it now, the library is unusable for any commercial application. I thought it was a great match for a project of mine, but there simply is no way to actually use it with that license.
The LGPL requires that the end-user be able to replace the library with an API-compatible library and still run the program. That's arguably impossible with an iOS device, and certainly impossible if you don't provide at least object files that can be relinked. Even with object files (or source) they'd have to pay to register as an iOS developer.
I suspect the majority of developers using the LGPL don't realise, or perhaps even agree, with this intent, so you're probably unlikely to be sued, but it's what the license demands.
I didn't say that using the LGPL would give you the same possibilities as BSD. Sorry for the answer that only half fitted the question, in context of iOS and App Store, you are right.
If the author wants to use a GPL license, its his decision. But using a viral license in a support library is harmful in any case, as it destroys all ability to use it in a program that incorporates other pieces of code that might not be compatible with the GPL.