Actually there is a checkbox you can click when managing your listing, titled "Lock on bedroom door". It's in the "Home Safty" section. I would think that the status of this checkbox should be displayed somewhere in the listing, so guests should be able to inspect this before hand. I had guests that didn't care for a lock, and a guest who made me install a lock, even though I didn't have that checkbox checked! I hadn't thought about it before hand, probably because of the implied trust that the review system creates. When I preview my listing, there is no indication on whether there is a lock on the door. So I think this is actually Airbnb's fault. Hosts see the checkbox and assume guests will know what to expect, but the lock status is not visible in the listing, so different guests arrive with different expectations.
If I had to guess why that is, I'd say Airbnb probably doesn't include this in the visible amenities section, from fear that it will turn it into an expected standard. It's tricky, because in some cultures the social fabric is very different and locks are not even used at front doors, let alone bedrooms. Then in other locations they may be considered a standard feature. In any case, not every host would likely be bothered to install a lock if it became an expectation, so it would result into fewer hosts and hence into higher room prices (see supply and demand), which ultimately means less business for Airbnb and a less sustainable business model. So hosts and guests are left to solve that on their own. But it can result in disputes and loss of revenue that platform pacticipants shouldn't have to deal with.
