Of course; those are freedoms 0 and 1 guaranteed by the license. It seems like "use" sometimes means "include in a downstream software package and redistribute modified versions of".
My point is that just having the GPL doesn't guarantee that modifications will be contributed back or published, even without violating the license.
Right, GPL only requires contributing forward, to the downstream users. It is only through creating a culture of working upstream and or reaching out to downstream redistributors that a project gets any contributions back, including financial contributions.