Mechanical Turk is Insecure
medium.com
medium.com
[1] http://mkronline.com/2013/09/17/how-to-make-money-with-mecha...
What is the work-around/alternative? Well written post.
A new account doesn't have all AWS services enabled by default, so the impact seems minimal to start with. Not sure if you can lock this down further as I haven't looked into it.
All in all this is not perfect, but "securer".
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.
You're right that challenge work does a reasonable job of screening tasks from search engines, though!