A private room means you get a door. Maybe the door locks, maybe it doesn’t. Do not be surprised if it does not. The privacy part comes from the door.
This seems to be lost in translation with people.
If you want to have a lock for privacy then you should look into getting the entire place instead of a private room in a shared situation. Which type of Airbnb is right for you? Shared Room or Private Room or Entire Place.
A host should not be penalized in the review with lower stars because you didn’t know that a private room doesn’t guarantee a lock on the door and you preferred one. Airbnb: Location the category that isn’t fair to hosts for guests to grade.
If you would like to ensure your things are secure then make sure you lock the residence. Your room will be inaccessible if thieves can’t get into the property.
I personally will never have the Airbnb rooms in my home have locks. 
- I don’t offer candles in the room anymore since I have found them left burning in the room when guests have left for their adventures.
- I have found space heaters left on with no guests around and I will go in and turn them off since they can cause a house fire easily.
- I have had to force my way into a room to prevent a guest from sneaking in other people in the middle of the night. Airbnb Hell: The Worst Guest I’ve Had To Date.
Since I have two rooms in my home, whenever a house rule has been broken, each guest likes to blame it on the other. I have only had one person own up to their behavior.
I would have a space that locks for guests if and only if it was a separate space that only they occupied so that if anything happened they would be forced to take accountability. I would also have a security deposit.
So the takeaway from this information is that if you want a door that locks, think about getting the entire place. If you find yourself needing a private room because cost is a factor, then ask the host before you book if the door to the room locks. I mean if you care so much then take the time to get the information you need to ensure you are comfortable instead of showing up, being disappointed/concerned and then leaving a bad review.
Stay safe out there! and Lock the front door! 🙂
Julie-Noted 