Google "The Black Mirror Colnemere", you'll find plenty of reference to people "guesting" on what was by then a no fishing SSI. Sadly, there are plenty of big names in carp fishing circles that have made their names (or cemented/enhanced them) by "guesting" and/or moving fish from one water to another. Amazingly, plenty even admit to such activities in books, magazine articles, videos etc. I'm actually surprised that more have escaped prosecution. This particular case, arguably less serious than some, could open the floodgates if successful. Whilst I don't know the individual concerned, nor wish him ill, this case probably needs to be successful for the benefit of angling as a whole. Its failure will be a nightmare for riparian owners as the likelihood of police involvement and successful future poaching prosecutions diminish if he gets away with it.
However, before folks start castigating carpers as the root of all evil, similar illegal activities go on with regards to other species. Plenty of similar tales to be found amongst the barbel and predator worlds. Just look to the distinct possibility that match anglers were responsible for Yorkshire barbel growing legs and ending up in the Ribble many years ago. Look at the distinctly unnatural spread of zander and catfish between otherwise unconnected waters.
It seems that many actually laud such activities, seeing the perpetrators as some kind of latter day Robin Hood type figures. It's in a similar vein to those that still celebrate the likes of the Krays.
One thing is for sure, many waters would not be as they are today (or were, in some cases) without the distinctly dubious activities of a minority.