The replication wizard, when used in basic mode, does not set the destination dataset to read-only by default.

Once the task is set up, it's necessary to edit the replication task and change the destination set read-only policy from IGNORE to SET.

If the User Guide is to be believed, opting out rather than opting in to read-only is the correct behaviour. This makes sense as you don't want the replicated dataset writeable by default.

Refer to this forum thread for more information

https://www.ixsystems.com/community/threads/replicated-dataset-is-not-set-to-read-only.86957/