That is because they are competitors, so this becomes an anti-trust case.
The scenario I'm talking about when you bring in a company as a partner. For example, we work with TCS and the contract states that we can't hire their employees and that TCS can't hire our employees, at least not to work within the partnership. We partnered with another company and as part of that contract we are transferring people them with the promise that we will not hire them back for a year or two depending on their position.