Just to add my $0.2 worth I look at it this way.....

Salmon migrate to saltwater and grow to maturity and then return to their freshwater birthplace to spawn...... Same for the Steelhead, but unlike the salmon, they return to do it again up to 3 times before dying.
This to me makes them a trout and not a salmon, since ALL true salmon die upon returning to freshwater and spawning.......

The folks that re-classified Steelhead just needed something to do to justify their 12 years of college I think.......

MC