I would think for many black South Africans it is similar to having a statue of Hitler in Jerusalem.
Cecil Rhodes was truly a disgusting man if you look at the things he did and they way he felt toward the native peoples of the continent. I may be wrong here but wasn't Hitler sympathetic to this guys feelings because Rhodes also had this intense love for the Aryans and hatred of everyone else? It has been awhile since I read anything about this guy but he really was a disgusting man.
As for Ghandi, most people like to remember Ghandi as this lovable fasting pacifist but again he also held the same racist feelings for the kaffirs" that most people did during his time and prior. History has a way of letting us forget about the filthy pasts of people and only the positive things that they have done.
Like you said Shaka Zulu killed tens of thousands of Africans, which is also deplorable. This young person not throwing poop on Shaka Zulu yet being upset with the white guy and the indian guy is probably the same reason you would bring up the black guy as an alternate example. I am sure if South Africa was always under the control of the native peoples that statue of a murderous Caucasian who hated Africans would never have been there because of the obvious pain it would have caused.
That young man should pay for his actions of poop throwing. Regardless of how painful that edifice of a notoriously hateful and bigoted man makes him or any one else.