Not simply "exempt", unless you're referring specifically to the trade window part. There is in fact a law making it illegal to trade on insider information. https://en.wikipedia.org/wiki/STOCK_Act Arguably it did not go far enough.

I just don't think Congress has much in the way of insider information most of the time. Antitrust litigation would be one example where they do, but what else? Company insiders by contrast almost by definition have insider information.