The SEC is currently pursuing a very expansive definition of insider trading (and are losing some cases because of it). I'm extremely nervous about stating definitively what they think is illegal insider trading.
That said, historically one of the things US* insider trading requires "breach of a fiduciary duty or other relationship of trust and confidence". Trading on material non-public information is not enough, there must be a relationship, there is a whole industry of people devoted to trading on material non-public information. They just have to get it without that breach.
So the argument for congress people (presumably, I'm not an securities lawyer) would be that most of the information they receive they do not receive via a fiduciary relationship or one of trust and confidence. Or that by Congress getting it, it becomes part of the public record. Though I can certainly see sealed/confidential testimony going the other way.
*This is an important distinction. In the US insider trading laws aren't about protecting the markets, they are about protecting shareholders. Its more about theft than market manipulation. European laws can have a completely different basis.