Not only so we focus on what just happened, but so we are primed to accept atrocities as "payback." That is why the debunked "beheaded babies" was important (before it got exposed as fake). This story is like the lies that Iraqis were throwing babies out of incubators, or Gaddafi was giving his soldiers Viagra and ordering them to rape women (note that this lie was tried again against the Russians in Ukraine).

It primes us to accept horrible war crimes by the Israelis, because "at least the Israelis aren't beheading babies!"