Well... yes. When you buy a product, you agree to an exchange of your money for the product in question. Once you've given your money to a person, corporation, or other entity, it's theirs -- you don't get to decide what it's used for anymore. If this isn't okay with you, you need to either 1.) negotiate terms in which you buy the product under the proviso that your money is used only in certain ways (this may well work with individuals, or work with corporations provided you're a large corporation yourself, but isn't likely to work in the case of, say, you buying from Microsoft), or 2.) not buy the product.

I don't think it's corrupt for corporations, unions, or whatever to make political donations. They have the right to attempt to promote candidates they like. However, I wouldn't object to candidates being required to provide a list of donors from whom they've accepted donations -- i.e., if you want to take that huge campaign contribution from the Chinese Communist Party, go right ahead, but the Washington Post is going to know about it. There is some value in democracy being transparent.