They are not undertaken because the parties distrust one another. They ensure that the terms of a voluntary cooperation already achieved are understood and will succeed the inevitable turnover in personnel that occurs. The government's role is to uphold contracts not to write them, re-write them or break them. Trustbusters, for example, have persecuted Microsoft for entering into restrictive contracts with suppliers and customers, like Intel, Dell and Compaq. Now, contract law has a long and noble history.
Trustbusters violate such law at every turn.
It should be obvious that contracts, by their very nature, are restrictive. The phrase restrictive contract does not symbolize a fraud it depicts a redundancy. Contracts specify whom you will trade with, on specific terms, at specific prices, over specific time periods. They restrict, as they must, the whom, how, what and when of your trading relationships. For those who doubt that contracts are both voluntary in origin and execution, while also restrictive in form and content, ask yourself what it means to enter into a marriage contract.