Probably for the same reasons we call Catholicism a religion, and Protestantism a religion, and Mormonism a religion.
Just because you disagree with something, or even think that it is evil, doesn't mean you can exclude it from the conceptual category it belongs in.
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If we as a nation fail to recognize that Islam is not merely a religion, but rather a sociopolitical ideology then it will only be to our own peril.
Islam is the antithesis of Democracy and despite our nation's valiant attempt to introduce Democracy to countries such as Afghanistan and Iraq, it will never be able to take root.
You need only open your eyes and see the destruction that is being unleashed on Europe as we speak now ~ what is in store for America is much worse.
The only sociopolitical systems where Islam is able to germinate are either autocratic or theocratic or a combination of the two.