Are there requesters who just reject hits for no reason? Is it just so they don't have to pay the worker? I really wish I had some feedback when given a rejection just so I can learn from it. I did contact the requester to find out why but have not heard back. Just curious.
It's very, very rare. Occasionally you will get rejections for inane reasons which will be frustrating, but it's unlikely the requester never intended to pay from the start. Often these rejections get overturned, sometimes they don't.
If a HIT seems to have been put together poorly/unprofessionally the chance of a rejection goes up significantly so avoid working on those.
The weight of a single rejection is often vastly overstated by turkers as even for someone who doesn't do may HITs it has little effect on their account. It does FEEL bad though -- so a lot of turkers tend to overreact. If the HIT didn't take very long/wasn't a lot of effort it's often not even worth worrying about and you're better off letting it go.