This is silly. Seeking out and watching porn is not the same thing as accidentally seeing porn because it arrived as spam or whatever.
With this line of reasoning you could say that when person A is caught looking at porn by person B, both should be fired because B happened to see the same porn over A's shoulder.
If you intercept porn and delete it, you're doing your job. If you retain it somewhere and go back to look at it, then you're just looking at porn, violating company rules.
You are correct (except possibly about the "silly" part). But if the official policy doesn't make that distinction, then I think there is still a problem.
With this line of reasoning you could say that when person A is caught looking at porn by person B, both should be fired because B happened to see the same porn over A's shoulder.
If you intercept porn and delete it, you're doing your job. If you retain it somewhere and go back to look at it, then you're just looking at porn, violating company rules.