The defaults are sane, and in fact the user here had to explicitly turn them off in order to do the thing they wanted to do. Once you reach into a configuration file and change a setting, I can't think of a software system in the world that protects you from your choice. Could you maybe name a few?
The user turned off persistence. There's no reason for a normal person to suppose that also means ignoring data that's in the system when the master comes up. The fact that the two are inextricably tied to one another in the Redis implementation is not the user's mistake.
What do you think a slave should do if it is told to replace its state with empty state? How about half-empty state? There's really no answer that's satisfying for every possible use case (certainly I don't want my slaves to refuse if I tell them to clear the database completely on purpose). And indeed you haven't given any examples of databases that try to do 'better'. I think that's because there aren't any.
