Why good laws are often cluttered with bad legislation

 When a solution is offered by a politician (or anyone in position of power, authority, influence, or control over others) why are good parts mingled with bad?

For example, let's go with gun control because it's often easiest to see the good and bad. The ultimate desire for gun control advocates is to take away guns. Guns are pushed as the problem that needs to be controlled. But that's not true. Any reasonable person knows guns don't shoot people, it's the person holding the gun who pulls the trigger. 

So the gun control lobby has to mix some truth in, like mental health problems or people buying guns illegally. By mixing some truth in with the lies more people buy into everything as being true, including guns being the problem.

The solution is then presented that gun control is needed. Part of the solution includes help for those with mental health issues. Part of the solution addresses how to stop people from illegally purchasing guns. And, since most gun control supporters also buy into the false idea that guns are the problem, restrictions on gun/ammo purchasing and possession are included.

Just a note on the illegal purchasing. Gun control advocates want you to forget the fact that we already have laws for this. And if these laws are brought up they'll brush them aside as being insufficient. Here's the truth, the laws are on the books but they just aren't enforced very well. The real solution would be how to better enforce existing laws. But, doing this wouldn't allow the politician to justify new and additional laws that would further regulate, restrict, and/or prohibit guns.

Why can't a politician just include the good parts? 

In the gun control case, the good parts would be increased funding for mental health and better enforcement of exiting gun laws and not including anything that infringes on the ability of law abiding Americans to freely exercise their Second Amendment right, which includes freely buying guns and ammunition without additional regulations or restrictions.

The reason is the politician aren't gaining any influence, authority, control, or power without the bad parts. Most politicians are also operating from a scarcity mindset. They fear any loss of influence, control, authority, or power. So they act in ways to preserve and increase their power, etc. This is why politicians often flip-flop on issues, get persuaded by lobbyists and other special interests, and, as it used to be said, talk out of both sides of their mouths.

If laws only have good parts (only those things that are needed which actually preserve and/or increase personal freedoms and rights for all) then the politicians gain nothing in terms of power and authority over others. 

But laws that are all bad (items in the law which reduce, regulate, restrict, and take away liberties and rights of others and increase influence, control, authority, and/or power for a few) are hard to pass. And if bad laws do pass in a democratically-oriented government and society, and the politician(s) gets called out on it, then influence, authority, control, and power is lost. 

So, politicians craft laws that are a mix of good and bad. The good parts "sugar coat" the bad so they are more easily passed. The bad parts are masked as needed, necessary, or essential when the truth is they aren't.