I'd rather vote for the following explanation:
those bloggers said think that instead of creating quality content, they can get by with only fancy image buttons (and other wonderful things like "oh, SEO, omgbbq!"), which are supposed to do the trick succeeding in their hit whoring trip.
unfortunately, being technically ignorant (as most of the clueless masses, .com target audience is constituted from), they are simply unaware about wordpress.com restrictions (banned JS, embeds, etc), which are not advertised by the automattic staff at the sign-up, of course.
and while addthis service is intended for the same yet lazy but hit-whoring sort of clueless masses, still it's supposed that they are, at least, users of some unrestricted, truly multi-user blogging platform (such as Blogger for instance) which does allow JS, essential for the stock code snippet to work as it's implied to.
if all I said above fails, then I'd have to believe in serious deficiency of either the blogs mentioned by tt, at the very least, or if not of their operators themselves. sorry.