Because Sasaengs are obsessed so buy anything and everything with the idols face on it so are a cash cow.
And sasaengs tend to know A LOT A LOT of personal information about idols that they have used to blackmail/threaten the idol/company with in the past. I remember there was a time a while ago (years and years ago) when I think it was Heechul was speaking out against a sasaeng and she threatened to release very personal information about Ryeowook if Heechul didn't take it back/apologise, which he ended up doing to protect Wookie's privacy.
so as much as I and most people wish companies would take a harsh line against sasaengs, sasaengs are also very fragile, petty creatures who won't hesitate to ruin someone's life if their fantasies are interrupted or changed in any way.
And how do the companies take a harsh line without risking their idols life/career?