Ken Livingstone knows that he is in trouble in this election, the last thing he wants is a tube strike just before the polls. The RMT know this too. That's why they have threatened to strike.
They did the same thing just before the 2004 election. Did Livingstone stand up to the union and do what is right? No, he rolled over and let them call the shots.
The unions bank roll Livingstone's campaign and now they dictate his policies too.