Why Is ♥ Emoji Allowed in Story Titles?

Why wouldn't it be? It's just a Unicode character. A quick test just now suggests that HN titles allow a pretty broad range of (if not, indeed, all) Unicode characters.

That's not an emoji, though. The symbol is ‘black heart suit’, and it's in unicode since 1993: https://www.compart.com/en/unicode/U+2665

Whereas emojis appeared even on Japanese phones in 1997, and were introduced in Unicode in 2010.