Law makers are exempt because they made themselves exempt, simple as that.
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.
If they're long the market and then boosting the market... I mean, isn't that what we want? You'd have to go further than a blind trust even and ban owning equities at all.