the reason "but her emails" is a used as a quip is specifically because of the tendency for that topic to be used as a redirection rather than acknowledging the actual topic at hand. there are loads of things like it that we also use all the time
hillary's e-mails, for example, have absolutely nothing to do with the stuff going on with flynn right now. there's no rhetorical reason to bring up hillary's e-mails here other than as a fallacious redirection