Yahoo! is publicly listed in the U.S., which means its officers have a fiduciary responsibility toward shareholders' money, including the profits. If it came out that Yahoo! management was accepting fines resulting in a 25% hit to profits in order to perpetuate a legal conflict with the NSA and FISC, they would get their asses sued by this time tomorrow.
Yahoo!'s management might well win the resulting public relations war, but they'd get their clocks cleaned in the shareholder lawsuit, up to and including ending some Yahoo! executive's careers.
People can't ask them to fight it out with the U.S. government under those circumstances.