Because that is what is often done. You have to learn this. They are coming empty right now and have been doing that for some time (otherwise there would be ťeď or nyní in the Czech sentence).
Therefore present simple, present continuous and present perfect are all possible translations here.
Ok... I think I get it. In spanish the logic would be rather the same. "Los autobuses VIENEN(vb.) vacíos(adj.)" Vienen can mean either they usually come, or they are coming, or they have been arriving.
Anyway... I wrote the present simple one, and duolingo said I was wrong. Just the foot note, but I already understand the logic, which is the important thing.
Thank you soooooo much! :D