Why do Christians say submitting oneself to Jesus is a "choice" when it's more like an ultimatum?

You do not understand the Christian doctrine that you are attempting to refute. Christian doctrine states that everyone is deserving of hell and stained by original sin inherited as the human condition from the time of their birth. Jesus does not make the ultimatum "worship me or burn" he offers the solution to the problem, that we are born with. It's more like we are already in a burning building and a fireman offers a way of egress from the building. If we take it we live, if not we die.