It's not multinationals in general, it's that Nestle has a horrendous ethical track record. For example, they pushed for powdered milk in Africa with absolutely tragic consequences.

http://www.theguardian.com/business/2007/may/15/medicineandh...

This is not like Microsoft using their market position to push their products. This is poisoning children for money.

In the long term they will lead you to something like Finnish food market ( which is kinda duopoly ). Long supply chains without any reasonable way to check origins of ingredients. Multinationals have also much more resources and incentives to hide and mask health and environmental effects of the food.

But worst thing is that to maximize profits taste is "optimized". And optimization doesn't mean good taste but rather "doesn't taste anything so that it won't annoy you even if you eat same thing every day".

just guessing, but multinational corporations might be viewed by some as not very helpful to humanity in long run, since they are capitalistic structures created to earn money to owners in first place. Morals usually comes a bit further down in priorities ladder (not saying anything particular about Nestle, albeit I vaguely recall some interview with their CEO where he stated all water on earth should not be free, but paid by common people, to... corporations)