To build upon what Rod said, it comes down to risk management.
If you allow a fishery for juvenile steelhead you have the risk of harvesting a juvenile steelhead that might eventually smolt and might eventually return, resulting in no "15 pound bullet" to provide an angler with a thrill, photo op, internet report, and a meal. If you don't allow a fishery, you risk introducing large numbers of hatchery juveniles into a system that will compete with the wild coho and chinook fry and parr for food and affect the viability of large numbers of wild salmon.
The latter risk has the greatest impact so fisheries managers allow a hatchery juvenile steelehad fishery in order to mitigate that risk. There impact of former is less, so fisheries managers mitigate that risk by timing it after the bulk of the outward migration.
Now I'm not naive enough to think that fisheries managers don't make mistakes however I tend to trust their judgement (informed by professional training and experience) rather than the opinion of BF who wants to catch a hatchery 15 pound bullet.
Some might think that they are doing a hatchery adult steelhead, or two, a favour by not harvesting hatchery juvenile steelhead, but this well intentioned, yet misinformed, position does no favours to the wild salmon that are infinitely more valuable.
Save our wild salmon! Kill a hatchery juvenile steelhead!