"USA Product for International Sales Only"...what does that mean, exactly?

I've seen this too when I've been out of the country.  Sometimes it's bad but not always.  The US has the most disgusting lax laws for what corporations can put into food and Trinidad & Tobago may have restrictions on some of these ingredients.  There are also laws regarding percentages of products companies can make for domestic sales versus exports.  The bottom line is, always read the ingredients (I always do) and buy the product that has the simplest list.

Quaker Oats is owned by Pepsi, which can't be trusted, so I'd buy something else in that case. 

I think YT may have it backwards, but I can't find definitive proof. Two cases where that label has been used before that I know. First, the item was made in the US, but no US taxes have been paid on it, so it must be sold overseas. Second, the item contains material banned for consumption in the US, but not in other places. It could be a mix of YT's example about domestic sales versus exports. Some sort of tariff dodging or tax exemption by labelling it that way.

I don't know for sure.  I do know US companies (like Nestle) have committed heinous acts selling sub-par products to overseas consumers.  But, for example, Mexican Coke is made with sugar because Mexico doesn't allow High Fructose Corn Syrup.  The taxes thing makes sense with labeling. You get the same thing when you buy stuff in the duty free shops at int'l airports.