Let me try to come at this from a different angle than many of the posts that are talking about specific laws.

Congress's job is to make the laws. They can try to make whatever laws they like. They can try to make a law that says the government will give them all a million dollars a year until they die, and either the president would have to veto it or the judicial system would have to find it unconstitutional to stop them.

In theory, they should be discouraged from passing laws that specifically favor them because there will be a backlash against them and presumably their party. Voters will know who voted for the "give Congress a million dollars" law, and those Congressmen will not be re-elected. It would also reflect negatively on the party, so not only would they not get re-elected, maybe a candidate from the other party would replace them.

The problem right now in America is that we have a lot of uninformed and ignorant voters who are devoted to the Republican Party. They will vote for the Republican NO MATTER WHAT. Gerrymandering has ensured that as long as Congressmen have these loyal voters, they will always win re-election.

So right now, many Republicans in Congress, especially in the House of Representatives (where gerrymandering matters) feel they can do whatever they want. Trump sure as hell isn't going to stop them. Their voters aren't going to turn on them. So why not try to slip clauses into laws like AHCA doesn't apply to them?

In the case of AHCA they got caught and had to take it out. But they'll probably try again, because politics has become so polarized in America that these GOP Congressmen think they are untouchable. They laugh at the idea that their district might vote for a Democrat, or that they might lose a primary when they can say they've been working to implement the GOP platform. And their supporters are too ignorant to realize they are being taken advantage of.