Teams can only veto the changes if it wasn't a safety issue.
Pirelli can officially declare that the changes are being made for safety and then they are free to do whatever they like. However, Pirelli insisted that the issue was an aesthetic one and the delaminations just made them look bad.
Pirelli, for the sake of saving face, did not want to declare their current construction as unsafe, so attempted to play politics to try to get a unanimous decision. That didn't work out.
Pirelli added that the tyres that will be tested by the teams in the free practice at the Montreal Grand Prix have never been used by the teams before. With regard to the new tyres, the problem of delamination has been solved by Pirellis technicians exclusively through laboratory testing. Delamination, which only occurred on four occasions and always because of on-track detritus, has never put the drivers safety at risk, but does risk harming Pirellis image. This is why the company decided to intervene.
http://joesaward.wordpress.com/2013/05/31/pirelli-states-its-case/[1]
It was based on this line that Force India and Lotus veto'd the changes.