Its not that any of these large companies should not have some influence; the problem is that having shit-tons of money is the seemingly only requisite to have ANY influence in government currently.
The term "oligarchy" has real and important meaning.
Take, for example, the last debate between Obama and Romney, where they took turns trying to show who was more pro-coal. You think it was because of campaign donations? Or all the coal mining/fracking jobs in key states like Pennsylvania?
But that is called pandering. Do I think that they were concerned with any of the coal jobs? No, I think they were concerned with public perception at the time they were visible to said public.
Politicians have Schrodinger-Syndrome: They only appear to be working/accountable when you're staring at them through the lens of a national camera. All other times, they are fervently working on interests which are not yours.
Insert {Halliburton, Exxon, AT&T, Comcast, BP, etc} into those brackets and you'll get a much better idea of the nature of political influence and lobbying in the U.S.
This is why I think all the bluster around Citizens United and the influence of money in politics is overblown. Because at the end of the day, a post-card listing the number of employees Exxon has in a Congressman's district is just as effective as a check.