In the end, the only workable way to allow free speech is to forbid anybody from altering the message between the sender or the receiver. Otherwise, nobody will be able to reliably transmit messages without assurances that the sense has been completely altered, even if we only allow "additions", even if we require those additions to be clearly labelled.
And why would we want to allow it, anyhow? For all those objections above, for all the problems it causes, including the fact it would eventually be co-opted by the companies themselves (see "Smart Tags"), it doesn't actually solve anything. For all the speech it throws away, it does not enable any speech that people did not already have the ability to do. You can already complain about a company. You can already discuss products. You can already post reactions to news stories.
The only reason annotation is even interesting is that it adds the content "directly to the page itself". But this is not sufficient to counterbalance annotation's costs. The reality is that while you have the right to speak, and the right to listen, you do not have the right to be heard. If nobody choses to listen to you, that does not give you the right to start defacing the speech of others. You have no claim on somebody else's web page, only your own.
The solution to the annotation problem is, mercifully, the one that seems to be winning on the Internet: Give people their own spaces, and let them say what they like. The purest incarnation of this is found in the proliferation of weblogs. Protect these spaces, and everybody can say their piece with confidence that the messages they post will be reliably transmitted to their receivers. This, not an anarchy where anybody can fiddle with a message, is the true path to free speech.