In the IRI, there is no filter that prevents anyone from cancelling another users 'attachToTangle' operation when the POW is in progress.
Why is that?
It's the responsability of the node owner to configure it's node properly. The recommended setting is to not allow remote-POW (therefore interruptAttachingToTangle is a NOOP).
If the node owner allows remote-pow, it is probably a good idea to not allow remote call to interruptAttachingToTangle. (using remote-limit-api parameter)