The reason is that somehow business has more political power than patients, doctors etc. That all comes back to the eternal problem of politicians having to have funds to be elected. So everyone in office owes favours to various people and organizations.
If campaigning politicians weren't allowed to accept any money from anyone at all (people or organizations) then a lot of the political problems in the US would just vanish I think. It's somehow so natural that americans tend to not even think that it's odd that personal political campaigns are paid for by donors.