I suspect this was done so that you could run distZip and not care whether an application or an library bundle was being created in subprojects for a multi-project build.
Maybe distTar and distZip could be special Tasks like build, assemble, or the like, and then each plugin adds their distAppLibZip and distAppZip tasks as dependents of distZip instead.
Not a huge limitation, but the conflict was a bit of a surprise.
-Spencer