Those are very difficult questions to answer. How much of Instagram's resources do they need to use in policing themselves? There are so many accounts out there that it is dubious that they can do so in a meaningful way. To me it seems that they would have to rely on people reporting such sites. And we all know how hard it is to evade those strenuous blocking devices that porn sites use:
Oh drat!! I am too young to see it. What can I do?
Though no Instagram fan I do worry about how this sort of lawsuit can affect internet usage in general.