I don't see a difference. Say you were for or against abortion. If there was an election and you could donate to the politician that agreed with your stance then you'd be giving him or her a kickback?
If I am an individual or part of a group of people that are against abortion and donate to lawmakers pushing for anti-abortion, it makes sense. I am contributing to the effort of trying to get that passed.
If I am a nursery or day care and am donating to the same cause, it makes sense because I have a vested interest in making sure more children are born. If more laws concerning anti-abortion start getting pushed through, my business starts benefitting from the new laws and the lawmakers enjoy a steady stream of money from their backers. It's still legal though because while it might seem fishy, it's speculation.
However, when those laws don't get passed and I straight up call out lawmakers saying, "I will no longer contribute if you don't pass these laws", it should be illegal. You are no longer contributing to the cause, you are asking to pay for laws to be made which favor you.