So, this is the poll you're referring to: http://www.people-press.org/2011/08/30/muslim-americans-no-signs-of-growth-in-alienation-or-support-for-extremism/
The actual wording in the poll is (in English, who knows what the poll said in Arabic, etc): "Suicide bombing/other violence against civilians is justified to defend Islam from its enemies..." (and then select one of Often, Sometimes, Rarely, Never, Don't know)
It's fairly bizarre to conflate suicide bombing specifically with an abstract range of things, violence against civilians. Violence against civilians could mean all kinds of things to different people, it's quite vague. The wording implies that only suicide attacks against civilians are relevant, not (military) suicide attacks against non-civilian targets, another thing to misunderstand.
Civilians itself is the key word, I guess, our assumption would be that violence against civilians is not permitted almost per definition, civilians being exactly those people who are not to be targeted. But clearly, Western armed forces have had a pretty tough time figuring out who is a civilian and who isn't in recent conflicts -- usually erring on the side of calling somebody an armed insurgent. We just define our problem away.
Next, the question whether an attack is justified. Under Protocol I of the Geneva Convention (caveat IANAL!), killing civilians can be legal in certain circumstances, you just have to try to avoid it, or not know about it (despite due diligence), etc etc. Calling that a justification of an attack on civilians is a bit twisted, but it's a legal framework. And of course it happens all the time, legally, and without any serious repercussions. The US hasn't ratified Protocol I, BTW. To be fair, the wording of "against" civilians sort of implies an attack where the civilian casualties are the objective, and not just involved. But that's a fairly fine point to make, people are being asked to answer a poll, not write a paper.
Defend is another fun word to toss in there, as I assume many subjects wouldn't consider your average terror attack an example of "defense". Or maybe they do, whatever, we don't know, it's pointless to argue about it.
Defending Islam strikes us as odd, because that ain't a country, but first of all the question/sentence was written by Pew, subjects were not given a choice of slightly rephrasing it (I guess their best option to deal with a false premise is DK or possibly no answer); second of all defending Islam isn't any stranger than defending freedom or the free trade and if anything it's less strange than fighting a war on terror or on drugs.
The final "its enemies" ties the whole thing up neatly, going back both to the point about who's a civilian and who's not and to the point about defense.
There'd be more to say, but I am all out of words.