Perhaps the specification is irrelevant, and it would be better to look at the actors' goals and incentives. Using the OP's example As a first approximation,

Perhaps the specification is irrelevant, and it would be better to look at the actors' goals and incentives. Using the OP's example As a first approximation, it's reasonable to say that the goal of the closed source software's producer is to make money, and the goal of the open source software's provider is to have people use it. The closed source software's author has no incentive to continue to follow the specification unless it advances the goal of making money; and the open source software's authors have no incentive to reject deviations from the specification if the result is a drop in usage. Postel's law still holds even in this asymmetric situation because that's where the incentives are.