If you extend this thinking far enough, they have a lot to gain. It's an extra dimension that Google can use to further refine their ad inventory against your interests. The theory is that you will see more relevant ads and Google will earn more because of it. i.e. if you like this video, and someone else liked this video, and they clicked on that ad, then maybe you'll click on that ad.
Perhaps on its own this single dimension doesn't add all that much value, but in aggregate and with a mass number of participants, it should.
Elon Musk's stated long-term goal is to colonize Mars. SpaceX is a step in that direction. It's an awesome long term goal in my opinion and always makes me think of early transoceanic exploration.
And yet, they can afford the hardware, which costs a minimum 5x as much as the software. It's not that they can't afford it. They choose not to. It's easy enough to obtain pirated copies of Windows in China. It's harder in countries with developed business environments and legal systems.
The salary argument doesn't work, either. Chinese PC penetration is something like 15%, so you really need to look at the average salary of the top 20% when considering affordability, not the country as a whole.
It's better to look at this as your usual supply / demand curve. If the price of the hardware and software combination were $100 higher (about the cost of Windows 7 Home Premium in China), then fewer people would buy PC's, but Microsoft would make a ton more across all PC sales.
I believe the lost revenue argument, but it's not 100% and it certainly isn't 0.
Perhaps on its own this single dimension doesn't add all that much value, but in aggregate and with a mass number of participants, it should.