CorePlus is not to be confused as the distribution.
It use is primarily to facilitate installations for those uses who do not have easy access to a network,
That being wireless only or network requiring firmware. As such it contains many extensions which are mount loaded, therefore not able to un-mount the CD, This is not new, the prior offering MultiCore one could not un-mount the boot media.
You may choose to boot without the included extensions, scroll through the boot menu. But then why would you choose CorePlus?
With TinyCore.iso one can umount the boot media as before.