I teach my students that raising a negative number to a noninteger power is very dangerous, because all algebraic rules stop working.
For example,
x^(n/m)==(x^n)^(1/m)==(x^(1/m))^n
is true if x>0, but it is trouble if x<0.
I think Mathematica does the right thing in this instance.
Try
N[(-2)^(4/5)]
N[((-2)^4)^(1/5)]
N[((-2)^(1/5))^4]
