The NSA system allows the government to monitor calls of innocent people, before they've done anything wrong. If sometime in the future a specific individual does something wrong, either on purpose or by accident, the government can then go back into the system and look through everything that person has every done online or with their phones.
In America, we aren't guilty until proven innocent, we're innocent until proven guilty. The government secretly monitoring everyone with a cell phone strongly implies guilt first. The government doesn't have a right to come into your home without a warrant, they also don't have a right to invade your privacy through the internet.