Natural human conscience is wonderful, until you start to get into questions about whether we should or shouldn't kill witches, whether it's okay to copy a new movie (what if it's 50 years old? How about 100?), when we said we were making this software free, did we really mean people could sell it (okay, but that didn't mean Microsoft? Okay, but it's only fair that we have access to their changes, isn't it? We didn't make this software so people could make baby mulchers with it!)
Making agreed upon contracts, whether for everyone or only for a small group, about what may or may not be done is the only way to make sure everyone is on the same page.