For one, US citizens are expected to pay taxes on money they earn outside the US, even if they reside outside the country. This, in and of itself, rubs people the wrong way. In some cases, Canada comes to mind, your US taxes are reduced by what you paid to Canada. This isn't the case everywhere. The more people-like corporations become, depending on your politics, the more likely you are to feel like the tax law should treat individuals and corporations similarly. Yes, this ignores a lot of the actual economic realities of corporations, but feelings and politics be damned :).
Two, the fact that the corporations in question are housed in the US means that somewhere along the line, there is likely some sort of tax break, credit, loan, law, or incentive program that they've been given to advantage them. After all, what large corporation doesn't have lobbyists and large building projects that can get funding from state and local governments. Some industries get subsidies directly from the government (The oil industry's subsidy being a popular one for political bile). Additionally, like people, corporations benefit from functioning civil services, roads and highways, and the protection of the military. These are all things that cost money (and no, lobbying dollars aren't going to cover that :).
Lastly, if you're more socialist leaning you likely wouldn't mind seeing a social safety net of some kind like a universal government stipend. Large corporate profits seem like an ends to a means in terms of executing that. I don't think a majority of US citizens feel like that's a great plan, but I've heard it suggested by some very left leaning thinkers.