I've found what I believe to be a bug -- although the Bugzilla people have initially indicated that the behavior may be intentional in opposition to documentation. It can be seen here, if you are curious.
As I'm posting this in General, I should mention that I'm a Red Hat Academy instructor. In the RH134 course content, it explicity states that emergency.target will mount / as read only (RO).
The official documentation, here, says, "In emergency mode, the system mounts the root file system only for reading, does not attempt to mount any other local file systems..."
In either of the following two cases, the system will reboot into emergency.target with / mounted as read/write (RW).
If an error is present in the /etc/fstab file (e.g.: nonexistent device or UUID referenced, nonexistent mount point, incorrect mount option specified).
If the systemctl isolate emergency.target (or systemctl emergency) command is used.
If systemd.unit=emergency.target is used in the boot loader, it will then mount / as RO.
It has been this way since RHEL 7.0. I've been illustrating, to my students, what happens when there is an error in /etc/fstab and you reboot. In those lessons I've simply used vim /etc/fstab to correct the error -- without ever remounting / to read/write.
Program Lead at Arizona's first Red Hat Academy, est. 2005 Estrella Mountain Community College