“When it comes to defending the freedom of others, to lie down and do nothing is an act of weakness, not humility.”
In the GNU Project we usually recommend people use copyleft licenses like GNU GPL, rather than permissive non-copyleft free software licenses. We don't argue harshly against the non-copyleft licenses—in fact, we occasionally recommend them in special circumstances—but the advocates of those licenses show a pattern of arguing harshly against the GPL.
In one such argument, a person stated that his use of one of the BSD licenses was an “act of humility”: “I ask nothing of those who use my code, except to credit me.” It is rather a stretch to describe a legal demand for credit as “humility”, but there is a deeper point to be considered here.
Humility is disregarding your own self-interest, but the interest you abandon when you don't copyleft your code is much bigger than your own. Someone who uses your code in a nonfree program is denying freedom to others, so if you allow that, you're failing to defend those people's freedom. When it comes to defending everyone's freedom, to lie down and do nothing is an act of weakness, not humility.
Releasing your code under one of the BSD licenses, or some other lax, permissive license, is not doing wrong; the program is still free software, and still a contribution to our community. But it is weak, and in most cases it is not the best way to promote users' freedom to share and change software.
Here are specific examples of nonfree versions of free programs that have done major harm to the free world.