So in this case we have to limit the fee for source, to ensure the user's freedom. In ordinary situations, however, there is no such justification for limiting distribution fees, so we do not limit them. They don't get anywhere this way. Free software is about freedom, and enforcing the GPL is defending freedom. When we defend users' freedom, we are not distracted by side issues such as how much of a distribution fee is charged.
Freedom is the issue, the whole issue, and the only issue.