I actually don't have any experience with the situation you've described. However, in the past when I've introduced my partners to BDSM and my own desires for it I have noticed a trend for them to be more comfortable in a sub position the first few times. I think this has more to do with responsibility and comfort levels than it does any natural inclination. Perhaps the ones you've been talking to have less experience? It seems to me that subbing is the easier position of the two (personal opinion only) because you really can't mess it up- you're not the one "in charge." Those are my two cents anyways, not sure it helped, but I hope it did- at least a little bit.