That's the whole idea of such barbaric terrorism. To send shivers down the spines of their poor families and others who are thinking of serving in Iraq. In our military or in an aid mission.
Terrorism is designed to scare people into going your way. Doing what you want .... in this case not coming to Iraq. In the case of Spain, voting socialist.
Same reason the Balkan and Chechnya Muslims filmed their snuff videos of grisly beheadings and tortures. To scare the other guy's military away or at minimum to take only safe ineffective actions, to hole up in his garrison a la the UN blue helmet idiots.