That's because the are a Farm Bureau, that also happens to sell insurance as a product to its members. You cannot be part of their insured pool unless you are a member of the Bureau, thus the membership requirement.
USAA is similar in that they restrict who they sell to, but they don't charge a membership to do it. Same as the OU Alumni programs though Liberty Mutual, and the AARP programs through Foremost and The Hartford.
Dang, I have FB and had 3 claims (all weather events) in the last 2 years and they have kept me? I cant shop around because no one would take me untill those claims drop after 3 years time. If FB dumps me I may be screwed.