This is a universal argument for doing anything that makes a profit in the short term. Usually, apologists for corporate immorality bring up fiduciary duty, as if it explains and justifies things. But the argument falls apart if you poke at it even a little, because it completely omits the other side of the argument.
Why doesn't Google start a restaurant chain, or start making PCs, or selling servers, or licensing software defined networking stacks to other companies, or monetize many of their other assets? Surely they must if they're supposed to make as much money for their investors as possible?
Why don't they sell search data on individuals to repressive regimes, so people can be hunted down, imprisoned, tortured and executed? Would make some decent coin for investors, I expect, lots of repressive governments would probably pay a pretty penny for it.
How about selling searches emanating from US government IPs to the Kremlin, selling tracking data linked to Google accounts, or otherwise monetizing espionage interests?
The problem with doing these things is that it affects the value of the Google brand, both internally to employees and externally to customers. If you can't hire the best employees or encourage customers to trust your software and services, then long or even short term decline is pretty much assured.
And how about swapping cause and effect? You're positing the tail should wag the dog - that the stock market should control what the company does. How about the stock market chooses to invest in companies that are doing things that it believes will make money? From this perspective, the company is free to do whatever, and investors are free to sell the stock if they don't agree. In truth, there's a mutual dependence here - the company can't just fraudulently give away investors' cash, but likewise, if investors knew what was best for companies, we wouldn't need CEOs or leadership of any kind - we'd just advertise choices to the investors / market and go with whatever pushes up the stock most.