Latched *not* locked. They aren't even called locks.
You were supposed to have a lock as well as a latch but many didn't.
There were better latches that could be locked using a key on the inside but they aren't as good as a mortice lock can be.
Just make sure you have plenty of ways to get out in a fire, the front door isn't likely to be one of them if the fire starts downstairs unless the detectors pick it up early.