"Small amounts" is certainly a very relative term given that the serving size can be arbitrarily small to misrepresent the amount of trans fat actually present as 0 grams.
For example, say I'm selling a product with the ingredients list "Partially hydrogenated soybean oil". If I label the serving size as .49 grams I could label the amount of trans fat as 0 grams per serving. Now this is obviously an exaggerated example, but it's easy to see from this example that a product could contain a non-trivial amount of trans fat with a specifically chosen serving size to label the amount of that trans fat as 0 grams (trivial). Therein lies the issue.
I learned in Nutrition class that there are standard serving sizes for various types of food. I think it probable that the FDA requires companies to use those standard serving sizes, and forbids them from choosing arbitrarily small sizes like you fear.
For example, one serving of meat is about the size of a deck of cards. If a company sells a meat product the size of two decks of cards, the FDA probably requires that the product be labeled as containing two servings.
Hmm.. That is interesting and comforting to know actually. I always figured there was some lower bound that a company would get called out on somewhere along the process if it was too egregious, but it's nice that there's at least some semblance of a standard.
For example, say I'm selling a product with the ingredients list "Partially hydrogenated soybean oil". If I label the serving size as .49 grams I could label the amount of trans fat as 0 grams per serving. Now this is obviously an exaggerated example, but it's easy to see from this example that a product could contain a non-trivial amount of trans fat with a specifically chosen serving size to label the amount of that trans fat as 0 grams (trivial). Therein lies the issue.