Rule #1 of good software design is that you take accountability for what a userbase does with your tools regardless of whether you have a legal need to, basically.
There's also something to be said for the fact that S-E not stopping what is essentially an exploit (trading 'unsellable' flagged items for gil) that happens to be fueling a profit margin, regardless of their actual intent or stated intent, shows a lack of good faith effort which is reciprocated by a lack of trust from the playerbase.
