Suppose you're a contractor who works full time at a particular company. This company is your only place of employment, and your contract doesn't have a defined term length (e.g. you're on till we don't need you anymore). Then you get all the miscellaneous perks of being a full time employee, but the company doesn't pay your holidays, time off, or health benefits. At that point, you start to look like an employee to the government.

This is why Microsoft and Google have defined contractor terms, and you can't come back for a period of time afterwards, etc.