Every time anything happens, ISIS claims responsibility. It's like free advertisement for them. Even if they had nothing to do with it they will still say it did for the publicity.
Have they ever claimed responsibility for something they weren't behind, directly or indirectly?
I doubt the press wants to give them more coverage, as a matter of fact, I think they avoid it as much as they can.
For example the Orlando shooter pledged allegiance to ISIS, but they didn't say that they were behind that attack.
They only glorified his act after he took the decision to do it.