We brushed on this subject in the recent copyright thread. It’s a shame, that private military researchers have such limited access to the collections. Years ago it was the norm, but today it’s no longer a problem making the pictures available for online browsing. Far larger collections have gone online – often with the help of volunteers.
If they insist on charging for the use of images, then at least make a low resolution image available online, so researchers can find the material without traveling. If they want to run a business with the material, then make it a darn business. Just as long as they remember, that they are in competition with plenty of public domain pictures these days – 95% of their material can be replaced (for publishing use) with public domain material from other sources.