I volunteer at both a county-run and a private no-kill shelter. I belong to both, but neither requires a membership to volunteer. I wouldn't mind if it did. Logically it would seem if you care enough to volunteer at a particular shelter, you might likely already be a member. Perhaps your shelter uses membership as shorthand way of screening volunteer applicants. They have a history with members and know they are who they say and where they live (for those solicitations by mail).