1970s in USA were a drastically different time than today. Information warfare did not really exist in a meaningful sense for the common person in terms of threats that they would have to consider realistically. Information was even hard to come by for the average person in the country back then.
Even ignoring those facts, if you're comparing that to email addresses being known or used today, I think that is also a very uninformed view of the world. The vast majority of the country even today does not understand what information about them is considered confidential, and what is not. I personally see this even among engineers from what are considered top schools today.
This is not a matter of intelligence. It is a matter of being informed. I don't believe that anyone is inherently stupid for being misinformed, especially in an age where information is used for warfare.