well said HC, i have suggest on another question that something like this be said to new users, just to be doubly sure people undertsand
its no use people saying others should 'just know', because they simply dont ... people are at different stages of their internet usage experience - some are old hands, some are very new, some are old, some are young, some are savvy some are not, and the upset from dotty shows how someone who has been using the net for many year, still does not undertsand ... so to be safe and sure, simply issue a warning as above and then no-one can complain or be hurt by it.