Er, that has been the Labour position for years.
They are a pro Europe Party and they have never promised a referendum.
Slightly more honest than promising a referendum, Cameron circa 2009, and then saying it is too late when he got into Downing Street.
Cameron needs a majority to give us the promised referendum (if he doesn't fudge it again), and every current poll says he will not get a majority.
Business leaders are mostly pro Europe, so Milband is highlighting his party's difference from the Cinsetvatives, to an audience who want to hear it.