Yea, but it's a Grand Tour and lasts 3 weeks, and riders in form at the start of the race might not be in form at the end and vice versa. Might Valverde lose a bunch of time tomorrow? Sure. But Jumbo still chased Nairo Quintana when he got up the road in last year's race, did they not? If Jumbo had had a guy in that move with Nairo, that guy would have been absolutely pilloried if they collaborated with Quintana, and rightly so. Valverde, 40 years old and not in great form though he may be, was on the podium of this race last year. That's not a guy you give free time to when they're only 3 minutes down. Had he crashed and lost half an hour in an earlier stage, maybe it plays out differently, or maybe it doesn't. Look at the last 40km of the Saint-Gaudens stage in 2009, with the group of 4. Efimkin does zero work, sits on the back for ages, then attacks... and everybody chases him down angrily. Luís León Sánchez is communicating with the other perseguidores, and at first they're working well but a little after that he's taking shorter turns, looking tired, a bit unsteady on the bike, having trouble keeping the wheel. They catch Efimkin, and then there are a few small digs and attacks, Sánchez is usually the slowest to react and riders are even trying to attack when they're in the position in front of him as they can get the jump on the others... but Luísle is bluffing and when he makes his move he makes it stick, and wins the stage. None of the others blame him or get angry with him like they did with Efimkin. They knew he was contributing and he was trying to work to close the gap and to chase down Efimkin, but he was on the edge... then they realised they got outsmarted, because Luísle played the game better than them. That was a stage where nobody had any GC aspirations or teammates in strong positions, so everybody was judged on their merits as contributors to the break. Woods could also have bluffed that he was more tired than the others since he was the one that had been away solo beforehand so he'd been on the attack for longer. If anything, the fact he didn't contribute when the group of 5 were rolling through fairly well together is the only thing that stopped him winning the combativity prize, hardly a wheelsucker's preserve.
Here, with Valverde so close in the GC, there are more factors. As long as they're collaborating, he's gaining. He is a winner out of today even without winning the stage because he gained a minute on the GC. Woods, unless he won the stage, was going to lose out on the stage; he would have expended his energy for nothing and enabled opposition to close in on his team leader. And since he didn't fancy winning the stage if he came to the line with Valverde, he had to find a way to get rid of him. Which he did.