As General Zod himself said in his (unfortunately losing) Preisdential campaign in 2008:
"When I first came to your planet and demanded your homes, property and very lives, I didn't know you were already doing so, willingly, with your own government. I can win no tribute from a bankrupted nation populated by feeble flag-waving plebians. In 2008 I shall restore your dignity and make you servants worthy of my rule. This new government shall become a tool of my oppression. Instead of hidden agendas and waffling policies, I offer you direct candor and brutal certainty. I only ask for your tribute, your lives, and your vote." (Source: http://www.zod2008.com/)
Given that it is unworthy of an interplanetary criminal to seek election, General Zod requires that the President kneel before him.