Just because one is incapable of mounting a good defense of something doesn't imply that what they are defending is automatically invalid. It's a really bad sign, sure, but a law could still be fine or even good despite the intentions of the law makers.
We're not talking average schmoe here. We're talking the federal government and lawmakers. They literally have to mount a good defense of their laws because they routinely get challenged and smacked down.
Go ahead and argue "cause religion" in front of the supreme court and see how far it goes.
That's not how it works. If it was how it worked, Jeff Sessions would simply go in front of the Supreme Court and say that the Bible requires Obamacare, and it would get struck down. The validity of the law is based on reasons the government could have for the law, or, possibly, based on the intention of the Congress that passed the law. It's not based on the Justice Department's defense of the law except insofar as their failure to defend a law affects the strength of argument before the Supreme Court.
The argument I'm making is that if you can only cite religion as a reason for a law, it should be struck down. There are a million other reasons why you can cite why Obamacare should be law.
If you can only cite morals for why obamacare should be a law then is it any different?