There are two reasons this feature is not enabled by default:
1) Experience from supporting millions of users, show that this causes more problems than it solves. Non-sophisticated users wind up encrypting much, much, more than they intended often causing real problems with other applications. More sophisticated users, like yourself, are much rarer but in most cases figure it out by themselves like you appear to have done. We can’t both have it as default and not have it as default, so we’ll have to go with the option that causes least problems and satisfies most users.
2) Since the feature is Premium feature, having it enabled by default would cause the application to change behavior in a subtle and non-obvious way when switching between Free and Premium, also causing support issues. Now at least a user has to enable it manually and thus be aware of the different behavior, and if it stops working when reverting to Free, a user will have a better chance to realize the cause.