As always, depends on state and local ordnances. If you are on private property you can always get hit with trespass, but the actual trash picking is typically legal. Many licensed PIs make their living going through other people's trash. In short, buy a shredder.
You are correct in terms of state and local ordinances, and I did do some more research and came up with the landmark case of CALIFORNIA v. GREENWOOD, 486 U.S. 35 (1988) where this very issue was argued before the Supreme Court: http://caselaw.lp.findlaw.com/cgi-bin/getcase.pl?court=US...
It basically says if your trash is visible, you can't assume privacy of the contents. If it's inside your garage, then it's off limits. Interesting case nonetheless.
As always, depends on state and local ordnances. If you are on private property you can always get hit with trespass, but the actual trash picking is typically legal. Many licensed PIs make their living going through other people's trash. In short, buy a shredder.