Flossing has it's place in fishing. The example you provide is a good one. If they are down stream of a hatchery, then they are most likely targeting fish that are artificially produced for consumption and should be removed from the system so they don't push out any nates (if there are any left on the system). Just because it may not be someones idea of "sporting" doesn't mean it isn't someone else's.

There are other times like on the Russian in AK where the Sockeye can damage their own survivability if too many escape to spawn. The system can only support so many fish and by the time they make it to the confluence, there aren't alot of 'biters' in the crowd. Flossing supports a good tourist fishery and succeeds in controlling the escapement.

Obviously, there is far more fisheries where it shouldn't be allowed and is an unacceptable practice.