Scenario... Given clearance to taxi from ground control which Included a runway hold short instruction for the active landing runway. We taxied to the hold short location. After clearing the last taxiway prior to the hold short location I switched over to tower frequency and we held short. After a minute tower called to ask if we were on their frequency and then then cleared us across the runway and told us to hold short of the takeoff runway (duh?). She then told me that I shouldn't have switched from the ground frequency. I have always been under the impression that the tower controller has to issue a clearance across an active runway. I looked through the .65 and the AIM and couldn't find anything conclusive one way or the other. Thoughts? This specific airport does a ton of things that are non standard, so I really don't know if they are right or not.