In a hair transplant, the hair that is transplanted is genetically resistant to balding (hair taken from the back of the head). But in women, their hair thins uniformly and from what I can tell, none of their hair is resistant to baldness, as is the case with men. So how is it that female hair transplants can work?
The type of hair loss that causes thinning hair all over the scalp is referred to as diffuse alopecia. More commonly found in women, those with diffuse alopecia are typically not good hair transplant candidates. Just as you suggested, there really is no donor area "safe zone". Typical causes of this balding condition include certain medication (such as for high blood pressure or atrial fibrillation known as A.Fib), a thyroid disorder, an iron deficiency, hormonal changes in the body, and scalp dermatitis or psoriasis.
Diffuse alopecia is not the same as androgenic alopecia or typical male or female pattern baldness. Androgenic alopecia is caused by genetics and some hair on the top of the scalp will be susceptible to DHT. DHT is found more abundantly in men than women which is why more men suffer from this type of hair loss. It is very important therefore, for a qualified hair transplant surgeon to evaluate each prospective patient (especially females) thoroughly in person before determining whether or not they are a good hair restoration surgical candidate. Women with hair loss in particular should be tested by a dermatologist to rule out any non-genetic causes of hair loss.