tsubaki
Player Valuation: £90m
Because the example your giving is utterly divorced from the reality of this situation. If this is about the Western Oppression then why aren't suicide bombers attacking military installations? One could argue the attack on the USS Cole, the attack on Westminster (to a certain extent) and even the Lee Rigby attack was aimed at the tools of 'Western Oppression' and, in the example you gave could be absolutely justifiable against an 'Islamist Oppressor.'
But the attacking of crowds of people out enjoying life with no links to the military complex? Really? That's justifiable?
Who is saying it is justifiable?
They attack weak targets - those kids on the buses in Syria, the poor Coptic Egyptians today, the concert-goers on Monday - because it is easy, it guarantees days / weeks of global media coverage (well, attacks in the West anyway), and because it doesn't markedly affect Western policy with regards to the region.
If they attacked a head of state, or carried out a mass casualty attack that killed too many people, then the chances go up that the West would reassess how it functions and make changes that start to negatively effect them. This is after all what happened after the Paris murders, when the horror of what they did, and the threat they represented, concentrated minds and brought about a little reconciliation in Syria.
That little reconciliation has put the SDF within three kilometers of Raqqa, and the Iraqi Army controlling 93% of Mosul. If we co-operate more than we have done (edit: and especially stop talking about attacking the Syrian Government) they are finished.