Even if we never send or receive an email, never access the web, and never make or receive a call, most of us can still be pretty accurately tracked.
If you carry a mobile phone with the battery charged, you can be tracked by GPS, or by triangulation from nearby cell towers (even if your phone is "off").
If your car has OnStar or a similar service, it can be tracked as well.
The government could use this to build a very accurate picture of where you go and who you associate with (e.g. what church, if any, do you attend? where do you work? where do you live? do you obey speed limits? etc.). This can lead to inferences such as ability to predict what political party you're likely to support, or whether you're likely to own a gun.
Even if you trust the current administration to only use this data for anti-terrorism purposes, the very existence of the database of "who associates with who" will inevitably lead to abuse by a few bad apples (e.g. local law enforcement) in the government.
