Festival Plaza isn't the only place you can launch a battle from: indeed, it's possible to select "Live Competition" from the starting menu without even going into the overworld, just as you would do to receive a Mystery Gift. That option lets you take part in battles that are in-person rather than over the internet, such as the big tournament venues periodically host.
In that particular context, and only that context, there's a bug that causes String Shot, Curse, and a couple other moves to freeze the game when you use them. No one knows why that bug occurs with those specific moves, or why it only arises in live competition mode, but the rules committee wisely believes it undesirable to have a state of affairs such that either player is capable of causing a crash on deman and thereby forcing a do-over of the battle. As such, they've decided to ban the moves in question from all tournaments played under the main VGC ruleset until such time as they can develop a patch for the game, which will fix those issues.