I got something similar here in Regensburg. Didn't apply, also as an American, I didn't realize that I could vote in anything.
It has something to do with Ausländer though, which I assume is why we received it...
They didn't explain anything, or anything about the different parties, it was just like- here, vote please.
Same thing, I could vote for 30 different individuals or just all of my votes for 1 party or a mixture.
I think mine did have an envelope in it though... not sure, I wasn't really interested, so I threw it out