The use of the harmonic minor with Am and E (or E7) is common, for example, in Flamenco melodies, in many "come-all-ye" tunes of the Down Derry Down variety (e.g., "Blue Mountain Lake" and "Days of 49"), and in blues constructions in Am (De Kalb Blues, for example). The use of the traditional minor scale using Am and Em is more frequently found in earlier Anglo traditional tunes (Greensleeves, Henery Martin) among others. But there are no hard and fast rules of class, in my opinion. As soon as you think you have a fixed set of rules,the next song you hear will violate them! :D
Many thanks, Stan, for clarifying the different varieties of minor scale!
A great deal of folk music is written by people who "work the chords" but are not conversant with the intricacies of scales and the musical theory behind them. As a result you get a lot of songs borrowing melodies and chord groupings from each other, and these develop into what are almost cliches in chord sequence and groupings.