I don't think Islam is being identified as a sole and total cause; it is
Islamism that is being identified as the possible the driving factor. Islamism is a politico-religious ideology. However, it must also be said that a prerequisite for becoming a violence-inclined Islamist is that you first become a Muslim. You don't find violent atheist Islamists, for example. You might even compare Islam to the concept of a "
gateway drug"; it is the pathway that
may lead ultimately to violent terrorist crimes
Furthermore, it is alleged (and I believe substantiated) that Islam's belief system and tenets easily lend themselves to an
Islamist interpretation. There are many passages in the Quran that justify, or even mandate, a mindset that prescribes violence against non-Muslims, homosexuals, adulterers etc until the entire world has come under the Dar al-Islam.
In addition, the complex interplay of religion and culture leads Muslims, both as a world religion and as a religious minority within Western countries, to consistently view themselves as being persecuted, under attack, victimised. The global Islam consciousness encourages a grievance culture (about the Israel-Palestine issue, about Sunni/Shi'a sectarianism, about Kashmir and about any situation where non-Muslims are present in "Muslim lands" ) that takes on a terrible and inexorable logic leading to a conclusion that violence is not only justified but mandated on both the political and religious levels.
Further to the above, and related to the above point, the decision of many parts of the European Muslims to isolate themselves from the host society's dominant culture, to look down on it and view it with contempt, to reduce opportunities for everyday and ordinary interaction between these Muslims and their non-Muslims fellow citizens, is yet another potentiating factor that creates the atmosphere within that community where literally thousands upon thousands of Muslims born in Europe to take up arms and join a group that makes war both on the country in which they were born and also against groups in the Middle East about which they know nothing but are willing to murder/rape/enslave.
The final factor is the attitudes of the "mainstream" Muslim community; their viewing themselves as first and foremost a part of the global Muslim Ummah rather than comrades of their fellow European citizens. And the "mainstream" Muslim communities sympathy for and agreement with this culture of victimhood and persecution, and finally their absolute refusal to concede even that there is a problem that exists and must be tackled.
So yes, it is relevant when a Muslim man goes crazy in a shopping centre and starts stabbing people (a tactic which ISIS has called on all Muslims in Western countries to do; that ISIS has ordered all Muslims globally to attack Westerners in any way they can, even with knives if they can't get access to firearms or explosives). In such a situation, it is absolutely relevant to determine whether his religion was completely irrelevant and he is just a crazy man, or whether he committed this not as a product of mental illness but in response to, and pursuance of, the Islamist ideology that has so cancerously spread within the Muslim community.
If he was a Christian or a Sikh or a Jew, it's open-and-shut that he's a mad man; there are no Christian or Sikh or Jewish terrorist groups he could be acting on behalf of, no global war and campaign of terrorism by groups from those religions against the West. But being a Muslim, it means we must explore his precise motivations and also whether he might have accomplices, whether he is part of a terrorist cell, whether he was acting on orders from an ISIS commander overseas or a recruiter or hate preacher here in the UK. Do you see now why it is relevant?