Likely a matter of 'for how long' though: almost everyone understands seat-belts being on for when turbulence is about to be expected or is actually occurring. Reports/'complaints' made on here are often because of the seat-belt sign being on for extended periods where it's deemed unnecessary.

Furthermore, it's a bit unclear whether the seat-belt sign was even on in this particular story here, but it appears it wasn't.
So, unless the seat-belt sign stays on for the duration of the whole flight, unexpected turbulence will just occasionally happen even with the seat-belt sign turned off.

(I keep my seat-belt on for the whole flight unless I happen to get up -which is rare-, so having the seat-belt sign on doesn't really bother me personally.)