Actually, lots of people and companies can legally trade on non-public information. See http://en.wikipedia.org/wiki/Insider_trading For example, it's not clear that Google can't trade stock based on what it observes its customers doing, even though that information is not public.

That said, Congress-critters are in a very different position. They're writing the rules. And, they keep claiming to be "public servants", so their use of non-public information should be at least as restricted as employees.