I don't even understand how that happens. At one point I was a consulting pimp for my dad, and multiple parts of the contract required me to attest that he would be doing the work and if he had help he would be doing the majority of the work and if his time on the contract dropped below 50% of the total the contract would be cancelled and there would be penalties.
There was no way I could have switched him with someone else without paying penalties.
Contracts have to be upheld by a court, to mean anything. I could write in a contract that you will be required to hand me your testicles if you miss the deadline, that doesn't mean I will actually go to court to get them. A lot of companies write aggressive contracts and never actually bother to enforce them, since it would be more costly than what they might actually gain.
Legally, yes - But remember that those provisions are there because someone tried that in the past and so they added legal provisions against it. And remember that those legal provisions are hard to prove; What is 50% of the work? How would they know if he'd had another developer submitting code?
There was no way I could have switched him with someone else without paying penalties.