The reason Google doesn’t accept the canonical tag as a directive is probably because they know many webmasters will screw it up. If you have a massive database-driven eCommerce site, and you’ve tried to get a developer team to implement, you’ve seen how it can ultimately launch with a ton of unexpected results. Examples I’ve seen: via templates, products were suddenly “canonicalizing” to the homepage. Page 4 of a collection suddenly canonicalizing to page 1 of the collection. Crazy, random results are always likely if not implemented and QA’d properly. When the tag was announced in February of 2009, I worked for one of the largest eCommerce platforms at the time. We wanted to be first to offer this, and we rushed it out – with many, many problems. I’ve always had a love/hate relationship with this tag.