It's permissive because the Eurotunnel terminal is a privately owned site which Eurotunnel can remove permission to enter at any point. Eurotunnel can and do close access roads from time to time, because access is permissive then they can do so without needing to apply for any permission from the authorities to do so. The user manual for OSM clearly says that Access for ferry terminals should be set to "permissive" and toll=yes.