This was a curious design choice by Amazon.

By choosing to use Amazon IDs as worker identifiers, Mechanical Turk gets all of the disadvantages of having anonymous strangers do jobs on the web -- lack of accountability, diminished incentives, malicious workers -- but none of the advantages of protecting workers, since at the end of the day, they're not actually anonymous anyway.

This is a non-issue in MobileWorks (and for that matter, oDesk and Freelancer as well). In these systems, everyone's explicitly working under a real name, which improves incentives to perform well, and makes it a whole lot easier to get good results through trust and honesty.