With a test, it might link up some functionality with "why" and pass, but then what happens if a business requirement just isn't a requirement anymore? The test will still pass. I'm thinking of something sillier, like a language that forces you to justify why for your code, and then regularly quizzes you if the business reasoning is still true. If anything changes, it rips out the code and breaks your site. :) So then you have to go in to fix it.

I'd also love it if this were applied to politics and laws.

It wouldn't be too hard to add such logic to your tests. If it proves useful, someone will no doubt turn it into a language feature.