They stuck to what they KNEW they could EASILY do: sneak into the USA and buy plane tickets.
Our "open border" policy to all-comers is a fatal achilles heal. We need to tighten that up, or face this again and again.
And we need to get over our PC "Barney" attitude (I love, you, you love me, we're a happy family). The radical islamists, (and that is 80% of some countries) hate us and want to kill us. We need to stop them from entering, and kick out any that do not prove loyalty.