<s>Right, just like the Linux kernel and OpenSSL.</s> Just because something is free doesn't _automatically_ mean you are the product. That said, I agree in this case - lots of free scammy apps are free because they make more money that way than selling the app.
Sure, but these free mobile apps typically are not open sourced projects.
Even so, a not-insignificant number of OS software is also a business strategy to buy B2B consulting services.
Linux and OpenSSL are not driven by philanthropy either. They are driven by technical passion. Weather apps not so much.