I believe that the sentence in Czech states that all the beds in that hotel are dirty. To say that "there are dirty beds in that hotel" implies that some of the beds may not be dirty. My answer "in that hotel the beds are dirty" was marked incorrect. However, I believe that my answer is more accurate than the one given in the course.
It does not say that all of them are dirty.
Your proposed translation is a different sentence: V tom hotelu jsou postele špinavé.
It is not just a different order, perhaps the clitic jsou obscures the difference, but one is "(v hotelu) (jsou) (špinavé postele)" and the other is "(postele v hotelu) (jsou špinavé)".