There are people submitting code with wrong licenses or no attribution. There are people just submitting for the sake of submitting - I dare github profiles for this. There are people who could need some feedback on their code, so that future contributions have better quality.
And it can be very burdensome for a maintainer, assuming he maintains within its free time, to perfectly communicate and elaborate on each contribution.
Also, maybe the project has a feature freeze because in the aimed architecture the same solution would be implemented externally.
Its just not that simple and people generalizing or concluding too fast are mostlikely in the wrong. Bad PR travles faster and further though.