Ha. I literally discussed this for 3 hrs today in African Politics class. US aid explicitly states that there is must be some sort of personal gain for the United States...a steady presence in Africa is a pretty key objective for any world power with the amount of unstable or authoritarian states.
So we're not being world police, we're being entrepreneurs and keeping our fingerprints on a region that has a lot of potential for future economic gain...if the aid workers can get their shit figured out. And it's not just the US that's extending its influence into Africa...China has been working a LOT with Zimbabwe recently. Not necessarily on a military level, but still.
Don't get pissy about it until you know the actual situation. which I'm sure nobody here on NS does...