Re: Why do we tell them this?
Right, so now we let them know how we found out where they were.
Oh, please. The number of ISIS masterminds paying attention to this story is probably very close to zero. The number of ISIS masterminds itself is probably very close to zero.1
This is not a cunning game of cat and mouse. It's a bunch of fanatics with guns (and other munitions) running around doing whatever damn thing enters their heads, versus an awkward coalition of all sorts of military and paramilitary forces constrained in various ways and frequently internally at odds, with a suffering populace caught in the middle.
ISIS combatants will continue to post selfies because there's little discipline in groups like that, and even in the most disciplined modern armies, under combat conditions people do dumb stuff.
1The government used to like to talk up the "terrorist masterminds" plotting against the US, too. But the vast majority of them never managed to create as much terror in the US populace as two guys with a rifle and a car did for three weeks in the DC area. And Muhammad and Malvo could have continued their activities for a long time, and spread them around the country, if they hadn't gotten greedy.
There are many highly effective potential terror attacks that are easy to mount, don't require suicide missions (so your trained operatives can move on and commit them again somewhere else), and don't require difficult-to-obtain materials. Coordinated arson, for example; a small team with some prep time could easily create a set of inner-city fires beyond the capabilities of city infrastructure in any large or medium-size US city. Do that two or three times and you'd get plenty of terror. But no, the "masterminds" focus all their efforts on explosive clothing, because they're unimaginative idiots who can't be bothered to do any research. Fortunately.