How is that?

I guess the sensible response is "what bias does being paid by Elsevier add that working for free for Elsevier doesn't add?"

The external bias is clear to me (maybe a paper undermines something you're about to publish, for example) but I honestly can't see much additional bias in adding cash to a relationship that already exists.

Exactly. At least if the work is paid, the incentive to do it is clearer.