It is the same phenomena you see with the sympathy for the murderer on Death Row, while almost no one talks about the murder victim(s).
The sympathy for the terrorists, mostly Muslims, is because liberals see them as underdogs and reflexively root for the underdog. They're poorer than the West - it must be the West's fault, we deserved it, etc. So supporting the group despite the violence against us is seen as a moral strength, and supporting their grievances allows them to attack their own society using the plight of the underdog as the reason.
In the case of Muslim terrorists, it is assuming poverty or racism or colonialism is the reason, so it is an excuse for more money to be given to these groups (while the ones demanding the distribution get to look like heroes for distributing others' money). And they get to bash Christians in the West as would-be Taliban members in the process, by blaming all religious extremists as the cause of evil in the world, while ignoring the fact that Amish (devout Christians) and Buddhist monks aren't blowing people up while fundamentalist Muslims do so on an at least weekly basis somewhere in the world.
In the case of death row murderers, it assumes poverty or racism is the reason, lets have more welfare programs and racist affirmative action programs.