I can see both sides. Personally, I don't think it wouldn't bother me in the least, but I also couldn't imagine moving furniture in somebody else's house.
Are they close friends? Maybe they just feel that comfortable with you that they have assumed you won't mind.
I think your only options are to either speak to them about it, or else put up with it. I do hope they at least have the courtesy to put them back where they originally are.
Really, when a person stays at a holiday home they don't really spend that much time in their rooms anyway, do they? I wouldn't think you would spend enough time in there that one would feel a need to move things around. I know when we go to the inlaws holiday home we are barely in the rooms as we're outside at the beach, ect from morning to night.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.