German Border Patrol

After a business trip to Frankfurt, I was cruising through German passport control on the way home. As per usual, I expected the usual humorless, suspicious, authoritative agent to greet me. I went up to the counter, and say "Gutentag", to which the PP guy replies, 'oh, do you speak German?" I have to answer no. He sternly demands "Why not? You are in Germany, are you not?" While I stand there and look a bit frightened, he and the other PP guy in the booth look at each and start laughing. The second guy goes on to ask me like 3 questions in German, and then they laugh every time I fumble to understand what he is asking - but I can't tell if their laughter is "haha, we will toss her ass in the brig" or "haha those stupid Americans are such chumps". Finally I tell them I have no idea what they are asking, but whatever it was, I didn't do it. Then they both laugh again (still no idea why), AND I got a stamp (!)....and bewilderingly I was waved on.....