It's using you as human labor to label data. That's why you have to solve multiple. It's annoying but greatly helps accelerate research and development since data is king.
That's actually not cool: recruiting your time under false pretenses to create value for themselves.
It's not as bad as you're making it out to be.
If that's what I'm helping you do then tell me that and let me opt-in. Maybe I want to contribute. But, don't instead throw up a road-block to the service I'm actually trying to consume, under the guise of providing that service, then double my required effort to help you provide some other service.
It's actually implemented as a dark pattern. If you want cheap labor then use Mechanical Turk or otherwise. But, don't steal thousands of hours of your customers' time under misleading pretenses.
You should complain to the site that decided to offload their captcha costs to you.
Your relationship is with the site, and so it should be their responsibility to inform you of the "bill" you'll have to pay to Google for using the site's captcha-protected services.
And, Google is definitely employing a dark pattern here. This is designed to be a stealth tax, to go largely unnoticed by anyone, including the site operator.
Even Google's selling of the product doesn't make clear that users will be doing additional work; instead suggesting that saving the world will be a by-product of a normal captcha process. [0]