I guess it may also be for quality.
For example it's not inconceivable that one could write an audio transcription bot that is quite good and may be good enough for a lot of tasks, but still not as good as a human and may fail quite badly at complex tasks.
Once the market gets flooded with bots, the buyers will work out what's going on and start to lower their offers to cover the risk of getting a bot transcription. Then the real humans leave.
And of course, if a task can be fully automated then amazon would prefer to integrate it into their cloud offering and take all of the profit themselves.
Amazon has to deal with the quality of human workers, too. It shouldn't matter who is behind the work.
>And of course, if a task can be fully automated then amazon would prefer to integrate it into their cloud offering and take all of the profit themselves.
Humans are more likely to care about reputation than bot herders who can trivially flood the market with programs which will work 24 hours a day. As soon as you allow bots, it becomes 99% bots very quickly.
You would also get people doing some work themselves to build reputation up and then swapping in a bot.
It would be quite interesting to build a dedicated market for bots. Just let people upload scripts with a documented input/output format and then pay the developer a royalty each time it is run.
From Amazon's perspective it doesn't matter if the user is a bot or a user. An user can also work themselves to build reputation up and then start doing low-quality work to increase pay. If you can't create a market for humans, you can't create a market for humans and bots.
The selling point of MT is that you can get human tasks done, if bots chase the humans out they loose that. It's similar to marketplaces like etsy in that regard.