I don't know who is doing the reposting, but the only party that seems to benefit from that sort of thing would be reddit. Why would a 3rd party do that?
As other people mentioned above, karma farming for bot accounts. Reddit probably benefits as well, just like any other social network doesn't want to clamp down too hard on bot activity.
Reddit's terms says:
>You retain any ownership rights you have in Your Content, but you grant Reddit the following license to use that Content: [...]
Users only grant reddit to repost their comment, not anyone else. Are you claiming that reddit themselves are doing the reposting?