Because it's not about its tactical effectiveness, but its strategic effectiveness. You kill those two in a place where they were supposed to be safe, at a time when there was no event near, and most of those other 12,998 are going to always feel in the back of their minds like they could be next, no matter where they are. It shows paranoia, and damages event morale.

But even more, I'm talking about the targeting of civilians for terror purposes. The effect of fear based on the killing of a few is amplified greatly because it's done in a place they're definitely supposed to be safe.