I'm afraid it has to be like this. The whole idea is to stop people downloading everything they can, for the sake of it, without giving something back to the community. Thats why contributors and artists have free access. If we open up the posters to everyone, then we will get people downloading it all, just because they can. I know you aren't one of these people, but out of the 7,500 members, there are quite a few.
Also, we can't anyway. The restriction to members groups in the gallery is a mod for Coppermine. And it can only restict the whole gallery, not individual parts.
Hope you understand!