The value they're providing is to the forum owner, who has reduced spam-handling workload.
So the forum is providing you value, you are providing Google value, and Google is providing the forum value.
Sure, but if the public has to provide Google with free data, we should make laws requiring Google to open source the entire ReCaptcha training set.
Why?
Anything created with free work should be free in return. Anything created by the public should be available for the public.
To use an in-thread example - online forums are created with free work. Should all forums be forced to make their archives available for free download as well?
For free download? No. Should I be able to scrape them? Yes.
Yes.
It's not free though. You get access to the forum.
If it was free then you wouldn't be doing them!
And if you think that it's somehow good because it's mutually beneficial to train AI to better the future of humanity, don't. That is what their marketing department wants you to think.