Impossible to build release version of Android app with ProGuard and ZSDK_ANDROID_API.jar

I recently added the Zebra Scanner SDK to my Android app (via a ZSDK_ANDROID_API.jar dependency). Doing a debug compile (without ProGuard) works fine. But when I do a release compile (with ProGuard) I get all sorts of compile errors. I've created a post on Stackoverflow with all the details. You can find the post here: android - Can't compile release version of app with ProGuard because of certain .jar dependency - Stack Overflow.

Why isn't it possible to compile a release version with ProGuard?