To be honest, there is an opt in service, and this may sound rude, but its signing up for Tapas. The opt out is just removing yourself from Tapas.
You don't have to agree to these terms, and for the most part, most comics on this site aren't going to be their featured comics or heavily promoted given the huge amount of creators already on the site. This is only for if a publisher wants to work with you and your comic, and Tapas, if they see your comic as a gold mine, wants to have the chance to negotiate with you to see if they can make a better offer.
This isn't taking your rights away. All this is, is a business exchange to have more exclusive comics to their site to gain more traction, more readers, more users, and more creators.
But I can see how exactly people are concerned, angry, or outraged by this. This is a technique that can be used to prevent artists from getting with perhaps a better publisher or allowing their concept to flourish. But again, this is a dialogue you can have with Tapas if you are that concerned.
Let's wait a little bit more though, they just said here that they want to change things according to the feedback, and they were at least quick to respond and help us understand a bit more about what this "Right of Refusal" is and why they added it. So long as we keep this dialogue going, we can get to a place where everyone feels more secure