Wildlife refuges generally don't allow uses that are not in keeping with the purpose for which the refuge was initially created. For example, Browns Park NWR was established to provide waterfowl breeding habitat to make up for habitat that was lost when the dam at Flaming Gorge was built. Uses within the Refuge that don't specifically fulfill that aim have to clear a pretty high bar in order to be allowed. An example would be the campgrounds and related facilities on that refuge. The idea being that without public support for raising waterfowl in this particular area, it would be more difficult to secure funding during Congressional and internal Interior budgeting processes. The campgrounds help waterfowl by furthering this aim and, more importantly, don't hinder the refuge's ability to fulfill it's real purpose.
All that being said, I can see where some refuges would allow caches and some wouldn't. Hopefully by understanding what the local refuge manager has to consider would help you better sell the idea of a cache on a refuge or to better understand why it would be denied. I don't work for the FWS, but I do work for another Interior agency-one that is VERY friendly to cachers!