28-pound rainbow trout caught in northern Idaho would have been record-breaker

As far as I know, in California, excepting  a variety of Shasta rainbow ( which is now above the Dam anyway) if a natural rainbow can make it to the sea, it will go there.

Perhaps historically, yes.  But CA rivers are a lot different nowadays.  Regulated river flows resulting from dams and water diversions have convinced many rainbow trout that would normally outmigrate to sea to stay put and remain as resident rainbow trout.  The resident rainbow trout in the Sacramento River around Redding are a prime example of this.  The bottom-release dams on the Sacramento River provide cold, controlled water flows year-round.  These cold flows also promote an abundant food supply of aquatic insects.  We've essentially turned it into a wet dream for a rainbow trout, so they no longer have the incentive to migrate to the ocean.  After all, why risk your life migrating through crappy habitat in the lower Sac, Delta, and Bay that's rife with predators when you can just feast on bugs in the river and reach 20+ inches without breaking a sweat?  If the dams weren't there, spring snowmelt would flush the rivers and convince the fish to outmigrate downstream.  Water temperatures would warm in the summers and food availability would be reduced, thus convincing any rainbows left that they ought to head to greener pastures (the ocean) if they want to grow big and strong.

About a year ago, Peter Moyle of UC Davis wrote an interesting article with regards to the Central Valley steelhead DPS and their "threatened" status.  It's worth a read:
http://californiawaterblog.com/2013/12/08/are-central-valley-steelhead-really-threatened/?utm_source=Fish+Report%3A+The+Status+of+Steelhead&utm_campaign=Fish+Report%3A+The+Status+of+Steelhead&utm_medium=email

As far as I know, in California, excepting  a variety of Shasta rainbow ( which is now above the Dam anyway) if a natural rainbow can make it to the sea, it will go there.
Sounds like that Idaho Rainbow was caught way up near the Canadian border, so maybe those don't go to the sea. Also that Rainbow looks more like a hatchery bred strain. Maybe an offspring of a hatchery fish. Maybe someone else knows if these hatchery strains have had their migratory instincts screwed up. Although I've never caught a Steelhead above 17#'s IMHO true steelhead would have a more elongated body

That fish looks a lot like the "trophy trout" they stock in some Bay Area lakes (the Lassen trout). It's my understanding that most of these "trophy trout" are "triploid" females, meaning they can't reproduce. If you think about it, you never see "trophies" with dark spawning coloration or the telltale hook jaw of a male trout looking for action.

I did some investigating on this fish, and the NF Clearwater River.  Turns out the area the fish was caught is located directly below a large dam.  There is a also a fish hatchery just below this dam.  And then, I found this:

http://www.spokesman.com/stories/2011/oct/30/trout-living-large-below-dam/

This part of the Clearwater is renowned for large, resident rainbow trout that chow down on kokanee that spill out below the reservoir.  But due to the regulations in Idaho, all trout over 20 inches in length in the Clearwater River are classified as steelhead.  A steelhead/trout with an intact adipose is considered a wild steelhead, which are zero-retention fish.  That 28 pounder is most likely a resident 'bow that's been feasting on kokanee his entire life.