On the other hand, if somebody said "I'm here for terrorism" and the immigration officer laughed that off, imagine the shitstorm if that person turns out to be a terrorist.

For the individual employee the cost of wasting someone's time by escalating the case and detaining them is zero, the potential cost of letting someone slip by is realistically tiny but potentially huge

The point is that the situation must be really crazy if we reach a point where someone (mostly foreigner) saying "tourist" is being confused as to saying "terrorist". Airport are full of tourists, and exactly 0 person on the planet would reply with "terrorist".