I started following that rule a while ago too, I wouldn't trust some random off-brand food from amazon, but the brand name food has a fair bit of markup.
If the counterfeiters are willing to produce anything to make a buck, and amazon is willing to sell counterfeit goods as legitimate, it makes sense we'd eventually have counterfeit brand name cereal or canned goods.
A while ago my fiancee was having trouble finding some Eagle Brand Medicated Oil (it's this traditional asian stuff her mom rubbed on her), and I saw it was on amazon. It's possible they have changed their manufacturing processes slightly since her previous bottle, or it was different because it wasn't as old, but it just kept being every so slightly different in subtle ways, such as the feel of the bottle and even cap, the texture of the oil, the smell was a bit different, the box was a bit bigger, etc.
I honestly still am not sure if it's counterfeit or not, but that's just the issue, I don't have the means to check. My only choice is to buy from sources with trusted supply chains.
It's ironic, I was the first of all my friends to start using amazon excessively, and got them into it, but nowadays my fiancee remarks she found her workout supplement on there for a few dollars cheaper and I reflexively reply "Hell no! We don't put things from amazon in our body.
At this point I only trust them with things that can fail on their own without causing damage, or for products so complicated no one has made a knock-off yet (I'm waiting on knock-off brand name monitors). They went from this amazing cheaper alternative to big-box stores, to AliExpress with 2 day shipping.