What does this mean? How can an emoji not have an emoji presentation?

An emoji cannot be an emoji without having emoji presentation. There is a rifle character that resembles, in form and function, the other Olympic emoji, it's just not sitting in the set reserved for emoji. Any platform that wants to pick it up can use it, they can even put it next to all their other emoji if they want to. Apple, et al did not 'prevent' Rifle from being encoded. They just decided they wouldn't pick it up as an emoji for their platform, and the Unicode conference decided to move it to a different category based on that decision.

There are literally thousands of characters that Apple doesn't encode for their platforms. I haven't heard whether Apple will be supporting:

    Osage, a Native American language
    Nepal Bhasa, a language of Nepal
    Fulani and other African languages
    The Bravanese dialect of Swahili, used in Somalia
    The Warsh orthography for Arabic, used in North and West Africa
    Tangut, a major historic script of China

All of which were added to Unicode 9.

>The two characters will still be part of the Unicode spec, but they'll be classed as black-and-white "symbols" instead of regular emoji

http://arstechnica.com/apple/2016/06/apple-and-microsoft-pus...

I assume they reserve the unicode character, but anyone who wants to use it decides what it looks like (so the rifle could look different on different platforms, which isn't a big issue)

This is always true for Emoji. The platforms always decide what their presentation of emoji will look like, just as they determine what font they will use for the unicode traditional letters. In this case, Apple and others decided they didn't want a rifle emoji, so it was moved to the 'black and white symbols' section so that those platforms wouldn't be 'missing' an emoji, which has other technical implications.