This is the same tactic used by the Al Qaeda or something in Afghanistan.
Utter despicable, but raises interesting questions.
Many "western" state-sanctioned talking heads (and hence, the general public) are quick to jump into the banal "they're evil mindless thugs" bandwagon, but some people suggested that there is more to it.
As medieval as they are, the ISIL here to stay. Despite repeated attempts by multinational forces to destroy them, they remain stronger than ever. Similar to what happened in Afghanistan, the crazy militant group (not really applicable to ISIL, but whatever) is pretty much in control of the country, unlike the puppet governments we propped up. But, predictably, the "west" refuses to treat them like a legitimate governing body. So to attract our attention, they turn to wreaking priceless national treasures. Brutal, but obviously works a hell lot more than the standard "murder civilians" thing which is pretty practised by any belligerent state these days, one way or the other.
But let's face it, the "west" will never negotiate with them. Our governments would rather continue to fund ailing pseudo-democracies (read puppet states)...
And seriously, that's why there is no end to this s**t. Too many people are caught up with this simpleton NUKE ALL OF THEM SCUM mindset. Yes, they are evil, but people join them because they are desperate. Tribalism also plays a large role: people would rather side with scumbags with a shared history than foreign shit-heads. Yes, we are shit-heads to them because we spout platitudes about freedom and democracy while drone-bombing innocent civilians and supporting unpopular dictators.
So maybe we should leave them alone. True, they are disgusting, but I don't hear anyone suggest that we should invade nations such as Saudi Arabia for their 13th century BC mindset regarding people without penises. Plus, unlike those failed governments we insist on propping up, they can actually run the damn crazy country.