Is it really all about reward and punishment for Christians?

I understand that you may have trouble believing (or accepting) a particular concept. Nevertheless that does not change the facts as the Bible teaches them. God makes no excuses, and He pulls no punches that morality (keeping the commandments) is all about having a good life (what you call a "mercenary desire for reward or some cowardly avoidance of punishment"). Though the language God uses in the Bible is much more dramatic and much more graphic than your meager quote. To get the full flavor of what I'm talking about you might wish to read all of Deuteronomy chapter 28. Those who are Christians should consider those words LITERAL TRUTH.