Because they won their conference. How did they win their conference if the regular season doesn't matter?
What I think we are saying is that in both scenarios, the regular season matters. It just matters in a different way.
However, in fact, I think in the current and future (hopefully 8 team playoff), I would argue that the regular season maters more. Last year, there were WAY more games in the regular season that mattered to the national championship than in prior years. For OSU, every game after VTech still mattered last year. In prior years, every game after VTech would have meant absolutely nothing as far as the NC goes.
In hindsight, yes. But every major conference team with 1 loss thinks they still have a chance to play for a national championship. Unless they're in the ACC.