mTurkers get to pick and choose which hits they want to work on. They see a list of available hits, and each has a title, description, submitter, estimated time, and reward. The submitter matters to mTurkers because this allows them to see the hit acceptance rate (how often the submitter pays out on submissions, or how often they reject submissions).

So it's simply a fact of the platform that mTurkers know who they are working for.