They're doing it to spy on the rest of the world, which is something that they've done for their entire existence. It's one of the two major reasons they exist at all.

It happens that now the rest of the world is using the same crypto we're using, but that's not NSA's fault. Nor is it a major degradation over a status quo; the government has usually been able to "spy on us", it's only been a short time comparatively speaking that it was even possible for the average citizen to completely encipher their communications. Telegrams, for instance, were copied and read as a matter of course if they crossed international boundaries.