Many answers here focus on a debate on whether or not one SHOULD provide a lock. Regardless of the opinion on this point, there is no explicit requirement, and as hosts point out, the option to indicate if one is present makes it clear that it's optional.
However, an overzealous Airbnb rep may arbitrarily determine something different, and because of the terms of use, you really have no recourse to that decision.
I just had a guest cancel after checking in because she was a newbie and expected the room to be a hotel room within the house - en suite bathroom, separately locked door. She cancelled because of "safety" concerns - she did not feel comfortable having to enter a common hall to use her private bathroom, nor in the fact that she could not lock her belongings in the room when leaving. Ironically, she seemed to have no problem having unfettered access to everything of mine.
Even though I do not have the lock box checked, she specifically asked me beforehand if there was a room key to which I answered no, AND she checked in and messed up the room before cancelling, Airbnb still gave her a full refund - with no feedback from me - because the lack of a lock was a "violation of hosting standards" and a safety issue. More, I was given only 2 hours to respond to an non-emergency email from the case rep before they made this determination, which is why they received no feedback from me, as I was at work.
Of course, once a guest has been told that they will get a refund, unwinding that decision is nearly impossible, so even though Airbnb was clearly 100% wrong, guess who took the hit?