James- you answered your own question, the dominant 5 chord is borrowed from the harmonic minor scale. When in a minor key it is common to utilize all 3 forms of minor together. For example, in that example you gave the 5 chord is taken from the harmonic minor but the 1 chord is from the natural minor.
The reason for the dominant 5 vrs the minor 5 is that the dom. has more pull to resolve to the 1 chord (E7 contains G#, which is the leading tone to A)
I can't think of any examples where the natural minor 5 chord is used in a minor 2 5 1 prog., it's always approached as a dominant.