I think the 20 inch thing is a management device. Any rainbow over 20 inches caught in a river likely got that big from going to sea so is likely an anadromous steelhead. Hence, you have to mark it on your card and you have a 30 fish yearly limit. Under 20 inches and its managed as a rainbow trout where the limit is two a day (in rivers), no yearly limit, different seasonal restrictions.

As Spawnout points out, resident rainbows provide another life history strategy to do battle with the variety of environmental conditions (and humans) that continually try to do it in. To separate the two for the expressed purpose of weakening protection is not only unscientific, but borders the unconscionable. You have a stock in such straits as to be listed under ESA, so you separate out a life history strategy that maximizes genetic variability so that it can recover, so that you can delist it. Makes so much sense.