Writing centers are unethical because they undermine instructors' goals.
Teachers give writing assignments to evaluate individual student learning and progress.
Assistance from a writing center tutor problematizes that kind of assessment. But that would be too easy, and it would only sidestep the more important issue of institutional accountability which these critiques seek to address. Their role as gatekeepers has the force of longstanding tradition, practical necessity, governmental support, and professional sanction behind it, and it should be no surprise that high schools, colleges, and universities take this role rather seriously. We must, I think, take this as an ineluctable fact of life. But by the same token, I don't believe that writing centers undermine this harsh reality in any meaningful way.