It's mostly a matter of choice. It's a lot easier to kill spawn them, so there may be a small cost savings to doing it that way.

Live spawning would allow kelts to be released to either survive and return again (not as larger fish, generally - they lose so much weight making the spawning run) or die and "feed" the ecosystem. If hatchery kelts survive at a rate similar to wild ones, then the return rate would be higher than the smolt to adult survival rate.

Sincerely,

Salmo g.