Well, looking into it...it might be a matter of common usage....
From another forum:
A knock comes at the door. Without opening it, you ask, "Who's there?". Should the response be...
"It is us!"
"It is we!"
or something else?
The latter seems more correct, but just sounds horrible. It also sounds more horrible when using the contraction, "It's we!"
Technically, "It is we" is correct, but you're absolutely right, it sounds awful. Frankly, I'm beginning to think it's nonsense to say that a subject, "it," cannot equal an object, "us." (The idea of the linking verb being an equal sign is kind of silly anyway.) I would rather say "It's us" -- or just stand there pounding until someone answers the door.
I mean - in the lifetime of anyone living - has "It's we!" been spoken?
In fact, has it ever been used?
I guess this is the difference between Proscriptive and Descriptive....^^