There's a section in the spec that says listCollections and some other commands that read should go to the primary, ignoring read preference. There's no justification written that I can find. If we relax this and say that all commands that read obey read preference, it simplifies existing specs and removes a contradiction from the Transactions spec. (If we leave "should-use-primary" in place, then there's a problem when you run listCollections in a transaction that was begun with read preference secondary.)