Because there are no grounds to delete the answer. I can't really understand the topic (ah, the joys of moderating plaintext gibberish), but it's from what I understand the "best" answer to that question that is currently available on the Internet. The fact the asker's been less than honest in attempting to wrangle the bounty reward out of it is not really relevant to the contents of the answer.
I feel that any action (including downvoting correct answers) other than separating the (apparently, good) answer from the (definitely, bad) answerer does not do justice to the greater good of giving great answers to great questions: the whole point of this website.
From my understanding of how things work here, by anonymizing the answer, the answerer would simply lose all reputation gained or lost from his answer; that wouldn't give the bounty back to Sorean, but from what I understand Sorean wasn't going to be awarded the bounty anyway without danRhul's downvote. This, however, is just my proposal for fixing this one problem. I cannot implement it myself and there's not mod agreement for enacting it. The agreement, rather, seems to simply ban the dude.
Truth of the matter, however, is that we (as moderators) don't have any tools to detect this particular kind of voting fraud short of the user spitting the beans out — which did happen here! I don't want the user to think "I wish I hadn't been honest! Next time I'm just going to keep my mouth shut."
"If you're going to commit fraud at least be quiet about it" is a message that makes a lot of sense, but I'd much rather send a different message: "Don't do fraud in the first place: we will bust you." I've contacted the team about it, we'll see what comes out of this.
In the meantime I encourage bounty placers to not let their bounty be automatically awarded.
Team replies back: