The lock is a tool of the user against third-parties.

Doctorow's Law: "Anytime someone puts a lock on something you own, against your wishes, and doesn't give you the key, they're not doing it for your benefit."

A lock you have the key to is nothing like DRM.

kmonsen is making the point that door locks on homes don't actually stop determined burglars. The only reason they work is because most people are relatively honest and aren't willing to take the step of breaking the lock (however trivial that may be), even if they'd otherwise happily open an unlocked door and walk in.

Not only that, but the fact that a door is locked almost certainly has legal implications for trespassers (though I don't know if that's really applicable to house door locks as much as it might be to locks on doors leading from public areas of commercial buildings to private ones).