Emoji were added to Unicode for compatibility with various mobile phones, so they would have a standard encoding. That's how Unicode ended up with the poop emoji for example - they didn't sit around thinking "what we really need is...". Since people really, really want more emoji, Unicode is sort of stuck constantly adding more. If you want to propose new emoji, the rules are at http://www.unicode.org/emoji/selection.html

Text symbols (as opposed to emoji) have different rules. Basically, the symbol needs to be used in "running text" (i.e. normal text), like "containers with [recycling symbol] can be recycled" or "he bid 2[club]". Traffic signs for example are not normally used in the middle of text, so they aren't encoded in Unicode. To get the Bitcoin symbol encoded, I needed to show that it was used in text, not just as a standalone icon. The full rules for symbols in Unicode are at http://www.unicode.org/pending/symbol-guidelines.html

For the snowman in particular, it was added to Unicode because it was a symbol used in the character set for Japanese TV broadcasts, see http://www.unicode.org/L2/L2007/07391-n3341.pdf

TL;DR: Don't argue "Why does Unicode have a poop emoji but no symbol for X?" - the rules are totally different for emoji and symbols.

Edit: does HN strip out arbitrary Unicode characters now? I originally had Unicode characters in place of [recycling symbol] and [club], but they disappeared when I submitted.

IIRC HN might have a character whitelist to prevent overloads of combining or layout-altering characters, and not have to worry about the behavior of newly added characters. There were some comment threads a few years ago that were just stacks of hundreds of combining diacritics that would crash some rendering engines and create odd decorated text on others.

Tom Scott has a great video about the history of emoji. Basically, some companies in far east countries were encoding these icons in various proprietary codes for their messaging systems. The ecosystem became widespread and consistent enough that the unicode consortium saw it fit to include these emoji in the standard.

The snowman, on the other hand, is a weather symbol for snow, I assume. It appears alongside other symbols for meteorological phenomena, so I imagine was added around the same time and with similar reasoning: http://www.fileformat.info/info/unicode/block/miscellaneous_...

Books aren't the only form of running text. Emoji are quite common in some forms of textual communication.

Emoji are widely used in text such as instant messages and forum posts.

They are mostly in Unicode for use in SMS, but there are plenty of use cases in other forms of text.

What about a text, or an article? Books aren't the only source of running text.

And what about email? I have to support POP3 (!); my customers would be seriously unhappy if they can't sent an email with emoji.

Heck, I'd be unhappy. I love adding emoticon and emoji and fun things to my emails.