To me it entirely depends on the motives for the action and the media outlet's propensity to highlight the POTENTIAL motives of other such instances.
If there's a reason to suggest that an individuals religious views are the primary, or one of the primary, driving motives for their criminal action than it'd stand to reason that it would be appropriate to highlight that fact in the interest of presenting the facts.
This is especially true in situations where a news organization will provide such information in other instances. If they'd suggest a killing was a "gang killing", if they'd highlight a shooter had "tea party leanings", if they're going to speculate that it was a "hate crime" because the attacker was white and the victim was black, then it'd stand to reason that if they're being a fair and unbiased provider of information to the public then they'd indicate that the terrorism was of the "islamist" variety. If the news organization goes out of it's way to not indicate potentially group/identity based motives in any such instances, then it'd make sense to not do so in an instance of islamic terrorism.