This is my understanding. A lot of B-list (and below) go to a lot of conventions to supplement their income selling photos and/or autographs, and some do fairly well at it. (It also gets their name and face out there again, if they haven't worked in a while but would like to.)

Also note that while SAG has a lot of very strict rules, and prefer to enforce them with cattle prods and pepper spray, there is always a way around any rule - if the SAG member wants there to be.