Some kind of "consent to be owled" thing? Sirius could be in a strict "don't owl me" setting (enchanted himself?), except for individual, pre-arranged owls, of whom Hewig was one?
Except in GoF he tells Harry to use random school owls, in case someone is tracking Hedwig.
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Except in GoF he tells Harry to use random school owls, in case someone is tracking Hedwig.