I agree. Propositions in Germany usually don't use the same construction in English and for the most part you can't directly translate them word for word like here on Duolingo. If you were to say "I am on the bus" and use auf/an for the translation then a German would think you were literally on top of the bus. Instead you say "Ich bin in Bus", which I think actually makes more sense. It's the same issue in French, my favourite being "Je suis fini".