Agreed. Carrying out a DDos attack is illegal, so these services are protecting sites against illegal activity. If we want to uphold the rule of law, protecting against illegal activity should be acceptable regardless of the contents of a website.

A physical comparison would be a security system for a business's property. The security company shouldn't be able to disable the system for a company simply because they don't like the politics of the people operating it. Doing that would essentially be facilitating a crime.