Obviously the "Next >" button needs that!
It just doesn't make much sense to trigger a "next"-submit with "return"-key, cause the natural place to use return is within the form.
But nobody ever edits the form if he just wants the next results of the unaltered form.
Of course one could use JS, but it's not really needed, at least in FF.
(addicted to the Perl Programming Language and ☆☆☆☆ :)