Why did Allah command the angels to prostrate to Adam, and why did Joseph's brothers prostrate to Joseph, given that prostration should only be to Allah?

Prostration is of two types: prostration of glorification and worship, which is exclusively for Allah alone, and prostration of greeting and honor. The prostration of the angels to Adam, and the prostration of Joseph's parents and brothers to him, were of the prostration of greeting and honor, and were permissible in their laws. As for the Islamic Sharia, prostration to other than Allah is absolutely impermissible; this is a part of the perfection of monotheism in this Sharia.