My mother is deceased these past 20 years, and my father is another anonymous number among billions, so I'm pretty sure I'm safe. But, since it is rhetorical, I'll go ahead and answer. Your parent going through things they may have paid for in a fit of drug/porn related paranoia is one thing, but you going through things that clearly do not belong to you is another. This is apples and oranges. Still fruit, but different variety. Your case is one in millions. Your parents seemed to do this out of spite, but again, their house, their rules. You as their child that they were raising are kinda obligated to obey them, with as much legal rebellion as you can muster. Your parents coming to crash with you after a disaster is a totally different scenario. They are destitute, and have nothing. You are taking them in, not raising them. They are still your parents. What is their's is not yours. You aren't raising them from childhood up, worrying that they might be on the dope or the needle, or beating off to animal genitalia. You are helping them. So, in essence, you don't have the right to do it to them, but they do because they are raising you, from childhood up, worrying about you and the choices you make. Besides being a parental duty, it's also their legal right, since most of the stuff you 'own' is listed on a reciept with their names and credit card numbers, with means that they really own it.
It's a very thin line when it comes to being grown inside one's parent's house, but again, you are in their house. Going through your things that you have paid for as an adult, with money you worked to make, is not legal. But, them going in your room and nosing around a bit or just being there is still their legal right. Their house, their rules. Now, should their be some diplomacy from both sides? Yes. Privacy is a courtesy, and it should be respected to a certain extent, but it's not a right, so it doesn't have to be respected, and any claim/argument otherwise is foolhardy at best. Would I respect my child's privacy? Yes, but out of choice, and only so far. Locks on your door in my house is not happening. And if I get the faintest idea in the back of my mind that you are doing some in any way wrong, I'm gonna do some investigative work. That includes searching 'your' room, and all the contents within it. And I wish my child would tell me to get out of their room! That would definitely result in a backhand that would make the Williams sisters jealous!
And besides...aren't you the least bit horrified that you might actually find something inappropriate? Little blue pills and Trojan Her Pleasures, and maybe some nice edible lube! Is that really an image you want being repeated in your mind for the rest of your life, haunting your dreams, and your own sex life? That would definitely take the award for 'Most Awkward Conversation between Parent and Child'! That would top the drug talk, the sex talk, and the dreaded masturbation talk!