People who have historically done these kinds of act don't care about IS nor are they devote Muslims. They care about support for Israel because it is seen as a further proof of the west mistreating Muslims. It's not about blaming Israel, it's about western governments doing things like the french did and first celebrating free speech after Charlie Hebdo, just to ban anti-Israeli demonstrations. That's a great recruiting tool to use on young person in a suburb already feeling mistreated by the government.