I always appreciated being able to opt out of the political fund when I was a member of Unison. I don't have a problem with TUs funding Labour- the bosses fund the Conservatives, after all- but members should have the right not to fund them if they don't want to.
In terms of "scabbing", my views depend on whether you're in the union or not. If you're not, crack on, none of my business. If you are in the union, though, I don't have so much tolerance. "Scabbing" undermines any strike action. Why join an organisation and then actively undermine it? If you don't agree with the way the organisation is going then leave it. But don't demand the benefits of the organisation and then stab everyone in the back when it suits you.