What happened is that a certain gardening recipe yielded good that you could vendor for more gil than the cost, and both players and RMT worked on a huge scale to maximize it. 15+ alternate characters and more all gardening and selling generated a huge amount of gil. There was nothing against TOS at all doing this, it was instead a problem with the recipe that SE should have nerfed.

However they chose to ban instead, and it caught a good amount of players it seems, because of the scale they were working on. To be fair, there's little way to tell whether or not a player with 2 accounts stuffed with gardening mules is using them for RMT or not. But in that case they shouldn't have banned so immediately, just flagged and reviewed each account on a case by case basis.

I think SE did this to try and remove the gil generated, but it backfired.