Ok, now translate that to an informal corporate-type situation involving only voluntary individual cooperation: A number of individuals make components that work together, which they sell to each other for assembly into final products. These people communicate and discuss how a particular political candidate is willing to introduce legislation that promotes their business interests. These people share this information, in turn, with their friends who lend them money to purchase parts and tools for their business activities. All these people have the choice not to support this political campaign, but they do it anyway because they want their product to grow and become more profitable.
In your situation, the corporate controllers spend the investors money on the campaign. In my example, the individual business people could borrow money or ask for more money in advance from their clients to get enough money to support the campaign. The big difference is that in the free-individual scenario, individuals must manage their own funds and political contributions - while in the corporate scenario, central agents can pool funds and make contributions out of the pool.
Maybe a better way to explain it is the difference between a couple being married or informally committed. An informally committed couple can behave in most of the same ways that a formally married couple can. Likewise, a formally married couple can behave either by coordinating their activities or simply lead uncoordinated lives as individuals. The point is that corporatism is not about a legal status, but rather about a mode of individual coordination with other individuals. Certain legal instruments facilitate this in some ways, but nothing prevents unincorporated individuals from coordinating their political activities on the basis of common business interests, for better or worse.