In our Terms of Service there's a section that explains our policy on giving away free review copies and desk copies of Leanpub books, and the occasional complimentary copy to resolve a customer support issue.
In these cases, we do not ask authors for permission to give away free copies of their books or courses, because we don't want to bother them with everyday aspects of helping authors make sales through the bookstore - that's our job, not yours!
We do this to help increase sales of your books and courses. Reviews from credible reviewers help spread the word about your book; that's why giving away some review copies is conventional in the publishing business. So is the practice of giving away "desk copies", for essentially the same reason: if a professor gets a chance to assess your book, in the end you might get sales to all her students.
Finally, on rare occasions we will give a customer a free copy to resolve a customer support issue. That description is intentionally vague, because these situations are not repeatable or predictable - if they were, we would solve the underlying problem, and there would be not any unhappy customers to deal with in the first place.