We don't accept that sort of rationale for anything else. I'll go shoplift some stuff (maybe steal some Nestlé products); other people are still going to steal whether I do or not, so I might as well not lose out on the profits.

Perhaps we should create and enforce regulations to prevent all companies from preying on the very human lack of self control.

That's a good point, from a purely economics point of view it doesn't make sense to stop. Having said that, Nestle pulling out of selling sugary products entirely would dent the market hugely, but someone else would eventually fill the gap. The moral aspect is a whole other question...

If every food producer were to follow that dictum, no one would have to take responsibility. So everyone would be incentivized to break the rules.

And if anyone wanted to begin new a behavior harmful to others, they would only have to ensure others also do it.

Decisions have to made on values, not whether someone else is already doing it. If someone's getting away with a crime, it doesn't mean you get to do it, too. It still means neither of you should be doing it.