But why shouldn't he lie? Surely freedom of speech includes the right to tell porkies about yourself?

Wasting energy on criticising this bloke diverts attention from McCain's rather more serious porkies. And the effect is to play right into McCain's hands. Makes it easy for him to present himself as the defender of a working man who is under attack by these nasty liberal media types. Diverts attention from the real issues, and helps to ensure that a lot of ordinary people are going to worry about being hurt by tax plans that would in fact benefit them (like the real Joe).