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The way I read this is that they're targeting shrines and tombs, including where these are attached to/combined with mosques, to fight 'idolatry'. I suppose if they found a modern shrine or grave which attracts devotees of whatever kind they might dynamite that, too. Obviously, it takes on bizarre forms not only there but also when they destroy old polytheistic remains wholesale that nobody venerates any more but only visits for historic interest. It's always ironic theologically that a strong God and faith should be able to easily withstand and overcome such temptations without the need for destruction.
More on ISIS not liking old things, including mosques.
http://www.rudaw.net/english/middleeast/iraq/060320151
http://news.nationalgeographic.com/2015/07/150702-ISIS-Palmyra-destruction-salafism-sunni-shiite-sufi-Islamic-State/
https://www.theguardian.com/world/2015/may/24/palmyra-syria-isis-destruction-of-treasures-feared