I normally don't either, but in this case the value is that ORG gets more political clout as a membership organisation than it would as a pure donation funded o

I normally don't either, but in this case the value is that ORG gets more political clout as a membership organisation than it would as a pure donation funded one. They get to say, 'we represent X000 members, of whom x00 vote in your constituency'. As you note, this does mean you have to check whether what they are saying continues to represent you ,and resign if not. But this hassle does buy you something. Perhaps the web site should make that pitch rather than making the donate option more obscure, though.