Because Eco, although the results are the same the motivation (whether you think it haram or not) also includes a different, added dimension.
Also the bombers often cite the need for invaders/crusaders to leave Muslim lands - thus they themselves overtly declare a religious reason. Corrupt or not they mention Islam, the tigers do not mention or invoke their religion. - This is why everyone 'thinks of Islam' when these atrocities occur.

From the Psychiatric Bulletin:

“In the case of the Tamil Tigers, though of Hindu origin, their motives, though nationalistic, have not been essentially religious……

A senior Arab psychiatrist in Gaza, Dr Eyad El-Sarraj, in an unpublished study quoted in a British newspaper (Silver, 2001) concluded (in reference to Arab bombers) that religion was a crucial, but not the only, reason for the phenomenon of martyrdom….

Prior to the ‘suicide’ bombing, a celebration would be arranged, which would be videotaped for subsequent display in a form resembling a religious ritual to sanctify the martyr before his community…..

…a moral and religious imperative based on it being honourable to die for God's cause in the face of oppression….”

http://pb.rcpsych.org/cgi/content/full/26/8/285

"Deaths in the Bible. God - 2,270,365
not including the victims of Noah's flood, Sodom and Gomorrah, or the
many plagues, famines, fiery serpents, etc because no specific numbers
were given. Satan - 10."