
In my opinion, not really.
You see, decreasing purchases in the US isn't likely to do a whole lot, especially in third world countries. The sales in those locations won't falter by decreases in revenue in the States. If revenue stays the same in those locations, the company will enhance efforts to increase sales. If revenue decreases in those locations, efforts to increase sales will increase. Either way, if the market is there, marketing will continue and sales will continue. That's just business. Is it unfortunate? Sure. Are those moms being duped? Absolutely. Is there a chance that they and their family will suffer as a result? You bet.
The same happens in the states. Consumers believe that they are receiving a superior product. They may even believe that it's better than human milk or at the very least, just as good. Many moms are convinced that they have low supply or don't make enough for a whole slew of reasons so they turn to formula. In the US, there is a taboo against breastfeeding. This may or may not exist in third world countries. Of course, in most places in the US, although tap water may not be ideal, at least it's a whole lot safer than what may be found in third world countries that ends up leading to illnesses and death. Here's the thing though, it's not the formula company's fault.
What?!?!?!?!
Hear me out. As a business, it's in their own best interest to make a quality product to promote future and repeat sales. It's also in their own best interest to market their product effectively to promote new sales. That's business, and that's what they are doing. Are their prices too high? Possibly, but if sales continue, it's in the best interest of the business to keep them there or raise them. Are they making moms believe that their product is ideal for babies? Of course, that's effective marketing. Are they getting their products and advertising into doctors offices who end up promoting either directly or indirectly their products? Absolutely, it's an effective marketing campaign and it promotes sales. That's good business. Is it moral? No, but that isn't their focus, sales are.
I'm not endorsing the company/companies. I'm simply saying that as a business, they are doing their job to make goods, earn money for those goods, and pay their employees and production costs.
The problem in the third world countries concerning the lack of breastfeeding in lieu of feeding formula they can't afford (that may be dangerous because of the water) is not the formula companies fault. The problem is the same problem that affects breastfeeding rates in first world countries. Lack of education.

Just like here in the states, those moms are booby-trapped.
Instead of getting irate and emphatic about boycotting companies, consider that. How about instead of the ultimate form of inactivism, (because really, boycotting like this is doing nothing but not spending money), you spend money to donate to and get involved in programs that help educate and encourage moms in third world countries? How about teaching these moms about hygiene and health, including how to make their water safe for drinking for their whole family, and teaching them that breastfeeding is their baby's best chance in their circumstances. This is how you do something about increasing breastfeeding and decreasing deaths due to unsafe water being mixed with formula. Teaching them that breastfeeding is healthy and ideal for their babies will save their family the precious funds they've been spending on a product they think will keep their baby healthy and well. Organizations do exist. Get involved. Start your own organization or join and support another. That is how we really can make a positive impact.