Postel's law is for when it's hopeless to get to a sane standard in the near-to-mid term, as with HTML and JS back in the day. Browsers had to implement ridiculously lenient parsing to provide a reasonable user experience on top of terrible markup and scripts.
In the case of binary formats, the application of Postel's "Law" is in practice only a workaround, used when there existed multiple different interpretations of a vague standard. If the standard had been defined properly in the first place, workarounds wouldn't have been needed.
I've seen Postel's Law being made official recommendation even — in an addendum document to a standard. The addendum got published only because the situation had become a mess.
But then, if your standard is broken, living with the problems caused by Postel's law is much easier than with the problems caused by not following it.
