The difference is the Executive branch falls under the Hatch Act but Congress does not. Article I Section 5 of the US Constitution authorizes Congress to make its own rules, etc., and they zealously defend that distinction. (That's the unspoken reason they use to not apply deathcare to themselves - don't want to cede any power to the Executive branch. That and they know the coverage will bite it.)
One may question the ethics of a Congresscritter harvesting addresses but, to borrow a phrase, "there is no controlling legal authority." Obama cannot use that excuse to avoid falling down the Hatch [Act].