Dog Vs Postman!

Why is it that postman deliver so early to homes that have dogs? 7:30 on a Saturday morning is too early to be woken by a crazed animal flying down the stairs to bark at the door! I'm sure the postman came later when I didn't have a dog. Do they make a note of all the 'dog' households and make sure they are first on the list for deliveries? Is it a case of 'I have to be up and working at stupid o'clock on a Saturday so I'll make sure you all get up too'? Somethin should be done - a new law perhaps. Is a lie-in too much to ask for??