Apparently the sign language interpreter at Nelson Mandela's Memorial was a fake. [Link] The sign language community is rightly outraged. I think I have a simple explanation.
My sister learned sign language and used to provide translations. It was a beautiful thing. I didn't understand it, but I enjoyed watching her graceful motions. It seems to provide extra legitimacy to the speaker, as if to say "What the speaker is saying is so important, we even want the deaf to hear it".
It would be natural at an event as significant as the Memorial service for Nelson Mandela to want to have this same symbol: graceful hand gestures sharing the message with all people. The problem is that learning sign language is very hard. It takes a lot of time and practice to learn and become competent. It's not like learning a new dance by watching someone else and picking up a couple of the moves.
There is a phenomenon referred to as "Cargo Cult", where a people imitate a more technologically advanced culture that they have been exposed to. For example, a Pacific Island people are amazed at the sophisticated supplies brought by airplanes to their island. After the colonizers leave, they might clear a runway and build air traffic control towers out of bamboo in an effort to bring more supplies, not understanding the complexity of actual air travel.
Unfortunately, someone involved in organizing the Mandela Memorial was practicing Cargo Cult Ceremony by adding the decoration of a man making hand gestures next to the speaker. It's kind of funny. And sad.