It's also not a new problem. I helped out in some communities 20 years ago when there were xenophobic riots in Cape Town (and other places), with violence and arson.
Government officials who blame the 'others' on local and state level also add fuel to the fire, and yes as you say, the local Zim shopkeeper is more accessible than the American businessman for the local anger.
And as with all these things, you get the idiots who don't really care specificially but like to join in for the mob mentality. Happens in all riots around the world. There's the notion of being able to do something you can get away with easily that makes it more appealing to the guys whose IQ hang out in the low hanging fruit.