Could it be because of a code or rule imposed by the Annunaki over their human slaves?

I lean towards this explanation. What better way to control the worker cattle than instill the idea they are gods in us. We were created as expendable beings, and their willingness to destroy us (as well as other animals) in the flood shows they have little regard fro creatures they feel beneath them, much the same as many humans today. Perhaps humanity got that trait from them in the DNA mixing they did to us.