Read-Only State field for a Release will not let the Start Sprint button show on the Agile Board
This is the expected behavior.
The Start Sprint button will change State to Current, so since the State field is Read-Only, having that button shown in Agile Board is against the Read-Only definition you have set.
Release affects it since rm_sprint is an extension of the rm_release table.
So if you are making the rm_release State field Read-Only, rm_sprint table state field also becomes Read-Only.