Amazon is protected by DMCA Safe Harbor provisions and doesn't have to act until they've received notice of copyright infringement on their servers.
There are many legitimate uses for BitTorrent, I don't see Amazon putting an outright ban on torrent usage on their servers. It limits their usefulness as a generic hosting provider to only allow certain types of traffic.
The article does point out that Amazon's Terms of Service disallow any piracy activity that may occur. So if anyone tries to use this for illegal purposes, they won't be up for long.
The potential for legal trouble is just too big.