Many spots have nice fish cleaning stations for use. It is much easier for transport and storage to fillet the fish on the spot. After all, they are not like bass which have a minimum legal length.
The requirement that rainbow trout must have heads and tails attached while in transit is due to the fact that there are some fisheries with length restrictions on harvested fish. Examples would be lake trout from Payette Lake (none over 30"); So. Fork Snake River brown trout harvest (2 over 16"); Henrys Lake tributaries (2 brook trout over 16"), Twenty-four Mile Reservoir (2 trout, none under 20"), etc..
The other reason for requiring heads and tails be attached is because in waters with anadromous fish (steelhead) a rainbow trout over 24" in length is classified as a "steelhead" whether it migrates to the ocean or not. If you remove the head and/or tail, we can't verify the length. We also need to have the tail attached so we can verify whether the rainbow/steelhead was produced in a hatchery.