That’s not true. My version of DAVdroid (0.6.11) definitely shows a no-write symbol beside my read-only address books and adds “(schreibgeschützt)” between the address book name and URL. I’ll happily send you a screen shot.
It may show this symbol, but it can’t set a “read-only” flag for Android because Android doesn’t have an “address book” table, only contacts. So DAVdroid is able to detect that the server-side addressbook is read-only, but it doesn’t have any possibility to prevent Android to write to this account’s contacts.
(I did not notice that this bug is calendar-only. Should I open a new one?)
Yes, please. But I don’t know whether it is even possible with Android to have read-only address books. But we can discuss this in a new issue because it’s not related to calendars (read-only calendars ARE possible).