Guild, Clan, what's the difference? They serve the same purpose and the terms are used interchangeably for the same function in various games. It's an unnecessary barrier to have one work and not the other, when /c doesn't even have a function of its own to justify keeping it separate from /g chat.
/a and /s often both work in games. One is All, the other is Say, they're used for local chat.
Why can't /g and /c be the same way?
You're in a guild not some clan cult. Get used to it.