I like to look at these things from the other side. Say you're a high-level politician and/or civil servant and want to find a way for somebody to pay you off for granting them political favors. What would you do? Paid speeches sound great. Hire a speechwriter for a few thousand bucks, then you just go and give it to them and collect your fee. That fee can be whatever you want, because who can say what a speech is really worth? Beats the pants off of running a charity for your benefactors to donate to, where it has to at least look like a decent amount of the money is going to some worthy cause. You can do whatever you feel like with your speaking fees. Your benefactors don't get to write off their payoffs as charitable donations, but that's hardly your problem.
I can't prove that that's how it happened, of course. But isn't that part of the appeal of the scheme? Who could ever prove that it was or wasn't a payoff in disguise?
It's like the conflict of interest thing. It's best to just not do it, as least while you're active in government and have the ability to subtly grant favors. Even if it's completely fair, standard, and on the level, it still looks dirty, you can never satisfactorily prove that it isn't dirty, and you waste a ton of time arguing about it, which just serves to cement in peoples' minds the notion that you are dirty.