Because most people have no idea how corporations function or how they're taxed, it's easy for politicians to make targets of them. "Corporation" sounds "big", and anything big is easily vilified. These people may not even realize that they work for a corporation or, as Brad pointed out, that their investments are tied to the profits corporations make.
When a government makes it too expensive for a corporation (or a company in any form, really) to do business in a city, state or country, it's only logical that the entity will look to move to a place where costs are less. All things otherwise being equal, there's more profit to be made if taxes or other costs are lower.
When legislators in Wisconsin floated the idea of taxing the profits of out-of-state divisions of corporations with locations in the state of WI, a lot of those companies started talking about leaving the state.
I don't know why anyone is ever surprised by this.