Well this is the same for most services, I've seen - including MobileWorks? If you register as a worker and you can accept tasks, if you meet the requirements.

It's avoided to some extent by us due to the way we issue training / challenge work before any actual work is given to new workers.

Not quite: in MobileWorks, work is assigned to private workers, so nothing is public. The same goes for CloudFactory, TaskUs, and the other non-marketplace crowd platforms.

You're right that challenge work does a reasonable job of screening tasks from search engines, though!