From a lecture by a doctor specialised in this sort of thing I learnt that the order of least risk for transmission between mother and child is:

1. No breast-milk.
   
2. Only breast-milk.
   
3. Mixed.

The order of 2 and 3 was the relevant point. If you give exclusively breastmilk the risk for transmission is less then if you generally use other food stuffs and sometimes gives brest milk. I do not remember the specifics of why this was the case. A law against breastfeeding might mean that someone who does not have HIV - a stigma in many socities. Thus with this law in place mothers might feel compelled to breastfeed in public in order to prevent stigma from attaching itself to them, while trying their best to give other foodstuffs in private. Thus creating greater risk for transmission.

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