It would be polite, and NetBSD, FreeBSD, and OpenBSD all have frameworks for companies to contribute money. Some folks just don't, but the license doesn't require it.<p>Companies that modify the code would do well to contribute that code back. It really isn't a matter of nicety in that circumstance, but a cost savings measure. Constantly having to patch after each release or, worse, being stuck because the community picked a different solution since the company's solution was not presented makes for increased costs. Losing the testing and expertise of the community should be incentive enough.<p>I have never believed requiring participation like a copyleft license would actually improves things. They don't want to engage and forced engagement really isn't going to help you. Let them learn the problems of going it alone.