So, while I realize Druids (and Bards) have largely lost all of their Celtic roots other than the names, this one is going to bug me. A Geas (particularly as a rule that dictates social or legal behavior, in particular hospitality) is from the same culture.
That Druids are the only casters who don't get access to this ritual is actually fairly offensive. It's cultural appropriation, no other way to describe it.