I've gotten several of those lately, and I just had to shake my head and say WTF? I'm very careful to read instructions and try to submit what was asked. The reasons provided have been nebulous and emails are not returned.
Scam assignments (signing up for offers, etc) = Yes Legit assignments (categorizing, etc) = Typically no Additionally, it sounds like you are not using TurkOpticon. Using this would benefit you greatly.
Have to agree with Andy. If the HIT is not legit (like my facebook page, vote for my photo, fill out this credit report, write a fake review) then the requester is probably not legit either. These requesters have nothing to lose, they get a few votes, reject the HITs and are never seen again (or if they hook a few suckers, re-post under a new name). Other requesters, who use mturk to produce meaningful work, want to establish trusted relationships with the worker community - it's in their best interest in order the achieve high-quality, usable results. Like Andy said, use Turkopticon to assess a requester's integrity, and ask here on the forum before diving all-in with a requester you do not have experience with.
Even the Turkopticon isn't fool proof. We all have to take risks and hope for the best. I've noticed some workers check the ones that are new (with no Turkopticon reviews) by seeing where they are from. If they are from a well known University, usually it's safe.
That's true and a good point. Academic surveys are generally a safe bet. They are bound to pay, if not you contact the University who will sort it out real quick. I don't many surveys so I never think of them.
When taking a survey that provides a contact email, I always note that with the date and survey name, so at least I can contact them if something goes wrong. I did get a reject the other day from some "academic" requester because I didn't provide my MTurk # or some stupid thing...I did, so maybe she's one of the few who just wants free work...they are out there. I was going to go back and do it again, but decided it wasn't worth it and I didn't want to work with her.