the captcha's from mturk (not a survey, right?)
I think they are - probably - looking at workers outside the US with more caution. This was because survey/academic requesters were worried about people outside the country responding to their HITS (when they were looking for US workers in their survey). Perhaps this has something to do with it.
At the end of the day, none of us 'know' what is up with mturk (ever). So, we can just speculate with you.
Nonetheless, it is good to let us know what is happening. You aren't alone. The more information we have, the more we can piece together things.
@Metallica has been in touch with someone at mturk about suspensions (and warnings). So, let's flag this for him to look at, okay?