As it turns out, some parts of yesterdays' post are not correct: The description of the behavior I expect was OK, but EGit behavior is not correct at all. Have a close look at the Git push spec and the sentence in bold:
The special refspec : (or +: to allow non-fast-forward updates) directs git to push “matching” branches: for every branch that exists on the local side, the remote side is updated if a branch of the same name already exists on the remote side. This is the default operation mode if no explicit refspec is found (that is neither on the command line nor in any Push line of the corresponding remotes file—see below).
And EGit does the exact opposite of that! Every local branch is pushed, no matter it exists in the remote repository or not. Thanks to Carsten Reckord, who pointed that out in my post in the Eclipse Community Forums. I'm waiting on another reply to my post, after that I'll file a bug.