http://docs.mongodb.org/manual/reference/read-preference/#secondaryPreferred

Manual says, "If no secondaries have matching tags, the read operation produces an error."

https://wiki.mongodb.com/display/10GEN/Driver+Read+Semantics#DriverReadSemantics-ReadPreferences

Spec says, "If there are no tag sets matching any secondaries, it reads from primary regardless of any tags provided." mongos and most drivers follow the spec.