They can apparently order you out, they can't just open your door. You do not have to assume you're under arrest. Maybe you should, but you have a RIGHT to be notified you're under arrest and you therefore have to be notified of your other rights. Police routinely violate these rights, but to say that you are legally under arrest and therefore have already forfeited rights simply because you're in the presence of a cop and that you should assume so is completely asinine.
It is illegal for a cop to do and they do it anyway and get away with it and you should assume this will happen to you, that doesn't make it legal.