Upgrade of Learning PHP, MySQL, JavaScript, CSS & HTML5 to 4th edition

Why can't I upgrade to the 4th edition of Learning PHP, MySQL, JavaScript, CSS & HTML5 for free? I have encountered errors in the HTML5 section of the 3rd edition but find that there is a 4th edition out which has substantially the same contents and has presumably corrected the errors but which looks to be a separate purchase. Why should I have to buy a new edition which is substantially the same to correct a few errors?
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