grammar - Why is it "This is he" rather than "This is him"?

Possible Duplicates:
Should I use a nominative or accusative pronoun when making comparisons (e.g. “I run faster than ___”)?
“Me” versus “I”

I've been told that "This is he" or "This is she" is correct, while "This is him" or "This is her" is not. For example:

Caller: Hello, may I speak to Bobby Tables?

Bobby: This is he.

Likewise, "We are we" is correct, but "We are us" is not.

On the other hand, you would say "I told him" or "I hate him" rather than "I told he" or "I hate he".

Why are linking(?) verbs and action verbs different in this regard?