Landorus using U-Turn is nothing like Thundurus using Grass Knot. U-Turn is all around a utility move that can be used in virtually any situation, and the simple fact that it takes out/weakens Celebi only adds to its usefulness. It's certainly not the answer to all of its problems, but the move itself opens up chances to build momentum. On the other hand, Grass Knot was completely situational and pretty much only for Gastrodon, and Thundurus had better options to run over it. Even when comparing Nasty Plot thund vs RP lando, they're two completely different sets. Landorus already has that immediate power due to Sheer Force so it doesn't need Nasty Plot to sweep, which in turn makes it harder in general to revenge. Of course there's the problem with getting the boost, but when comparing the two that part doesn't matter since you assume they've successfully grabbed it. If anything, those points actually work against anti-ban arguments.