Petitions are usually a means of communicating with people, or raising publicity, which in itself isn't a bad thing. The point of a boycott is to hurt the prestige and reputation and well-being of institutions that are harming Palestinians through their complicity with the occupation.
It's no good saying "ah, but you won't boycott the US". Aside from the fact that Iraqi unions (for example) are not asking UK lecturers to boycott anyone, whereas Palestinian unions *are*, a boycott can work in a limited range of circumstances, and one such instance (previously) was South Africa. Of course the same excuses were raised then by apartheid's apologists, who said that good old SA was being targeted unfairly, and it was all gesture politics etc. I can't vouch for it, but I like to think that some of the solidarity that was shown the victims of apartheid contributed to the international isolation of that regime, the application of sanctions, and the final success of the ANC. Only a little bit, I admit, but at least it was something.
Incidentally, you lot finger-wagging and claiming it will never work haven't yet answered the point that the Zionists are going mental over this - *even though* the decision doesn't even amount to a boycott yet. They did the same thing when the smaller AUT reached a similar decision and, through the usual range of bullying, diversionary tactics, smearing etc, eventually reversed the policy in a special conference.