Japanese people have the special affection for human-shaped objects.
With the influence of Shinto culture, they often regard human-shaped objects as resting or residing room for souls, spirits or gods/deities. Though this custom is becoming obsolete, some Japanese take old dolls to shrines or temples to pray for the repose of souls. Priests or monks recite a sutra and the dolls are cremated. These might be the reason why some people in Japan often talk as if Vocaloid characters were real human beings.
It seems some MMD model creators take their works as the very expression of their souls with this cultural and religious belief. This strong affection for human-shaped objects may lead to the reason why they strongly reject editing or redistributing their models. They don't mean to be selfish at all; we should respect their feelings.