If you're thinking of just one round of testing this is true. But if a 3rd party tester gets a reputation for being tough then everyone tests with another test

If you're thinking of just one round of testing this is true. But if a 3rd party tester gets a reputation for being tough then everyone tests with another tester. This creates a race to the bottom, where the is an implicit competition to get a reputation as the least stringent tester. This is essentially what happened with the credit rating agencies during the financial crisis.