You can, but you choose not to?
I followed your link, but it didn't really answer any of my questions. It did, however, provide an audio recording (14m33s in length!), so I gave you the benefit of the doubt and wasted some more time listening to it. It didn't answer any of my questions either, so perhaps I can clarify them by repeating myself.
How does “al-Dawla al-Islamiya fi al-Iraq wa al-Sham” turns into "Daesh"? An acronym is generally composed of the a concatenation of the initial letters of a phrase or sequence of words. That would result in either "DIIS" or "ADAIFAIWAS", depending on whether or not the initial letters of definite/indefinite articles are included in the acronym. It would not result in "Daesh" based on any common system of generating acronyms. Indeed, the "a", "e", and "h" in "Daesh" don't correspond to the initial letter of any words in the original name of the group, and there is no explanation of why they are found in this supposed acronym.
Why is "Daesh" capitalized the way it is, and not as acronyms typically are? For example, let's look at other acronyms. ISIS, ISIL and NASA, but not Isis, Isil, or Nasa. A key property of acronyms is that they are expressed in all-capitals. If Daesh is an acronym, why is it always written as Daesh and never as DAESH (as we'd expect an acronym to be written)?
Why should people use a foreign language to refer to a concept which is adequately represented using their native language? Since Slashdot is an English-speaking site, it seems unreasonable to support conversation in other languages, including Arabic. Learning Arabic in order to participate in discussions about this organization is an unreasonable burden, and communicating in a language in which one is not conversational doesn't seem reasonable either. When English-speakers are discussing this group among each other, is there any benefit from using a foreign language instead of English? Does this benefit outweigh the benefit of using the native English instead?
[I'd also mention something about the inherent absurdity in trying to create a Latin acronym from Arabic script, but Slashdot doesn't care for my non-Latin character set]