How is it private?
"By donating to a political party, you are basically trying to help get that party into power, and impose your own preferred policies on everyone else. How is that a private act?"
By voting for a political candidate, you are basically trying to help get that politicial into power and impose your own preferred policies on everyone else. How is that a private act?
Your lousy interpretation of the issue aside, you will probably find that donations above a certain size need to be disclosed; small donations do not. That's generally how this works.