> In that case it makes more sense to do a copyleft license because it will legally require all derivatives to also follow that open source idea.

That is a matter of opinion. I have put out some open source stuff under the form you mentioned, and it's always BSD or another permissive license. I view it as quite wrong to force my moral choices upon others, so I give others the freedom to use whatever license they like. In my case, that is what makes sense because of my own moral convictions.