I think it's about time for an explanation of why the law is this way....clearly people have got the oddest ideas about it.
It comes originally from the WHO code of the marketing of breastmilk substitutes, which aims to protect mothers and babies from the worst marketing excesses of the formula industry,all of which we have seen here in the UK, though not always recently. It includes reps pretending to be nurses and advising women to use formula; incentives to health workers to persuade mothers to use formula; free samples of formula to mothers...and yes, discounting.
Because the decision to use formula is not very easily reversed, mothers need to know the likely impact on their family budget if they use formula. In families where money is very tight, a jump in the amount of money needed to keep the baby on sufficient formula (because a special offer ends) might mean the mother has to save money by diluting the formula or by giving ordinary cows milk before the baby is ready for it.
This is a live issue in developing countries, where poverty (real poverty) is much more common.
It is not so much of a live issue here - 50p off a can of formula which is then restored the next week when the offer ends is probably not going to make a difference to families in the UK. However, the principle remains that mothers need to know accurately what it will cost to formula feed a baby. In addition, it is not good for a baby's sole source of nutrition to be affected by commercial considerations or ones affected by marketing - the cheapest formula that week may not be the right health choice for a baby.
Personally, I would be in favour of cheap, non-branded but quality-controlled formula, which was not advertised and which had no incentives for anyone. I don't want formula manufacturers operating helplines (bar the ones they should operate to answer questions on their product); I don't want them offering prizes to midwives and gifts to health visitors and treating them to lunch, or going into parentcraft classes to teach massage and all the rest of the things they do to promote themselves.
Consistently cheap, high quality formula, readily available and not advertised or marketed - that protects breastfeeding best, and it also protects the health of formula fed babies, too.