What's the ethics of (paid) proof-reading college admission essays?
Having a qualified reader provide more than superficial corrections to an admissions essay is a valuable asset, and one which is almost certainly required for top tier institutions. Many American students are literally surrounded by qualified readers with diverse perspectives, while international students may have fewer contacts with the requisite mastery of the language. If the only competent advice a student can obtain is from a paid reader, they are still only narrowing the gap with their American peers. I think there are only two ethical arguments to consider here.

First, is the paid service worth anything? If the criticism and corrections are junk and the service does not increase any student's chances at all and the service provider should reasonably be aware of this, then it's clearly unethical.

Second, does the contribution of the reader go substantially beyond what an American student would receive? Specifically, do the contributions of the paid readers potentially cross the line to misrepresent the abilities of the applicant? If the submitted essays are not representative of the, as you put it, story and voice but also of the English competence of the student, it could be a disservice both to universities and to inadequately prepared students.

If you are clear of these two issues, I see no ethical complaints. Of course, a generous person might donate their time, but that doesn't make a student who needs to earn a living unethical.