Once you start telling people how they can spend their own money, freedom is just a joke.
You cannot legally buy votes. You cannot legally pay to have someone killed. You cannot legally buy another person. Obviously freedom is just a joke and we should be allowed to do these things. Or, one could realize that freedoms among people are various balancing acts, and that striking the right balance is a good one. I don't think that you should be able to effectively buy a politician's vote. It's corrosive to our government, and our government is whom we charge with enforcing our notions of freedom. Hence, more freedom is preserved if we restrict this one; similarly with slavery, contract hit jobs, etc.