People you hire on MTurk don't generally work in the US, and have a small fraction of the carbon footprint of an average American.

Aren't most of those arguments here moot, since they also need a computer (+ networking) to be available on MTurk?

computer < human + computer

Of course not every computer has the same specs and footprint, but they should be in roughly the same ballpark.

A typical computer used by a typical person has only a fraction of energy use of GPU farms utilized to train DNN models. We're not at the point where you can pick an existing DNN off the shelf and just use it, you have train (at least partially) a new one for each new task, often many times.