Should the US Government be allowed to spy on its citizens?

No government should be allowed to spy without restraint on its citizens. But there is always tension between complete freedom and a government's duty to protect. There has to be a reasonable means for a government to gather information necessary to the security of the country. More and better information would have prevented 911, but on the other hand, we have no idea what other atrocities have been averted because the right information was gathered and applied in time. Outright spying without cause and without restraint is the behavior of tyrants.