Why are curly braces in functions not optional in C-style languages?

I'm sure this would mess up the grammar. For example there would be no difference between this empty function definition...

void empty()
{
    ;
}

...and this function declaration:

void empty();

I'm not sure that they did "choose" this. Rather, the ability ommit curly braces for if, while, for, etc. emerges as a natural consequence of the way they specified the grammar. The grammer forbids it for functions probably because of the old-style function declarations.