Someone explain to me why we're supposed to abhor this

I mean newscasters and political commentators donating money to political campaigns. MSNBC host Keith Olbermann has recently been suspended without pay by his employers for having donated $7,200 to three Democratic candidates (apparently NBC's corporate policies prohibit news reporters, but not "opinion program hosts," from donating money to politicians). I'm not going to argue that NBC's decision on Olbermann should be any different--he did break the rules. What I disagree with is the rule itself. I keep hearing, from pretty much everywhere, that donating money to politicians by journalists "raises conflict of interests questions," but so far no one has explained exactly what this conflict of interest is supposed to consist of.

Those who argue that large donations from corporations and individuals should be banned usually do so because they believe that it creates the possibility of "buying policies" by the rich and powerful. Suppose we agree with that argument; what we can do then is put a cap on the maximum amount a single donor is allowed to contribute, and all is well. Once that is done, what is the point of singling out journalists and banning them from donating altogether? I fail to see how Keith Olbermann could be buying anything from the Democratic Party by bribing them with seven thousand dollars, for Pete's sake. Sometimes I hear that if journalists are allowed to donate, it creates a threat to their journalistic independence. Again: how, exactly? I could see a threat to journalistic independence if the Democratic Party were found to have made donations to Keith Olbermann's retirement account; but where is the conflict of interest in a situation where it's Olbermann paying the Democrats? (Or Sean Hannity paying the Republicans, or whatever.) Is the concern here that if Olbermann is highly partisan then his newscasting will be biased and misleading to viewers? Suppose this is the case; then how is forbidding him to contribute money to the Democrats going to make things better? If a TV show is partisan and biased, making it impossible for its author to donate money to politicians isn't going to make it any less partisan and biased. In fact, I can see a compelling argument for how it could make it even more so. Suppose Olbermann has a burning desire to help the Democrats in whatever capacity he can. One way he expresses this desire is through donating money to them, the other one is through trying to "smuggle" pro-Democratic propaganda on his TV show. The latter, however, is difficult because of his employer's evaluation standards, so he's treading carefully in order not to get fired. Now suppose he finds out he can't donate money anymore. Isn't it possible that he will now try to channel a bit more of his desire to further the Democratic cause through his TV show than he did before?

On a related note, I think we're concentrating on campaign spending too much. Research shows this is not where corruption happens in American politics. Vote buying happens through barter deals being struck between legislators and various other entities (deals of the sort when, say, a corporation rep tells a Senator "So your vote is pivotal for bill A and you're trying to get perks for your district in exchange for doing what the party wants you to do. If you make your vote on bill A conditional on passing bill B, which we happen to like a whole lot, then we'll place our next investment in your state.).