Agreed, it's hard to understand this making any financial sense. If you're looking to do substantial seeding there are low cost unlimited bandwidth vps' that specialize there. If you're simply looking for a delayed but fast torrent download there are services that essentially transfer torrents to files on their server for you to download. And if you simply want to download things fast & on demand, there is always nntp.
Imagine if someone failed to set a max seeding amount and forgot about their EC2 instance for a few days or weeks. It'd be easy to transfer hundreds or thousands of gigabytes (1TB out = $150)
AWS is one of the most expensive ways to buy bandwidth out there. To do worse you have to look at high end dedicated & managed colos and bad deals at big CDNs.