There is no single way the baby can be positive if the parents are both negative. I'm telling you from a point of simple genetics.

The reason they do it is because they are never 100% sure that the father is really the father, and they don't want to women to be put in an awkward position. If you are 100% (and I'm not saying You, more the general you), sure that the father is negative, i would flat out refuse to have it. There is a risk (very very minimal), as it is a blood product, and hence comes with any of the associated risks with blood transfusions etc. If you husband is negative, there is NO REASON AT ALL to have it, there is no risk to the baby, so you are having a completely unnecessary injection that puts you at a slight risk.

I'm A-, but my husband is positive, so i have it during my pregnancy and afterwards (they test the babies blood types, but i know from looking at the blood types of DH family that there is very little chance that my kids won't be positive). I believe (my personal opinion here), that Rh neg women (with positive partners) should have anti-D, as the risk is very small.

But that being said, if my husband was negative, i would tell them no way, and then i would ask them to explain their interesting concepts of basic genetics