In at least one tourney that went on in my area, it was held on a public Delayed Harvest trout stream, that yes, was stocked by the WRC (with taxpayer money). It was not a derby-type event, but a fly fishing tournament with members from various "fly fishing teams", etc, etc, present.
Several fishers in the area weren't too happy about the situation either.
Another one happened on the Cherokee Indian Reservation tribal waters on Raven's Fork River, which is a heavily-stocked water too, and open to the public with the purchase of a license (or day pass). So yeah, they were fishing stocked trout.