I still don't quite get it; couldn't they have just mandated the usage/inclusion of trans-fat in product, and let the consumers avoid it? By the same standards, we're still letting people smoke and drink.
For one, if you have less than 0.5 grams per serving in your product you can label it as 0 grams of trans fat. This is a problem because serving size is arbitrarily determined by the producer of the product, which with a small enough portion size can still leave trans fat in significant portions while labeling it at zero grams. Other tricks exist like using different types of oils (trans and regular oils) in different manufacturing plants and calculating the average amount over all locations the product is produced. The other issue with this is that even relatively small amounts (relative to how much food we consume) can be very detrimental to your health.
The other issue is related to the first and that even consumers that want to avoid trans fats can have trouble doing so if they don't realize the first point I made, and the only way to avoid that is to scan every ingredients list for partially hydrogenated oils.
Simple fix. Require transfats to be reported in milligrams instead of grams
In any event I don't think that banning trans fat was the only way to solve the problems I listed. Though I do detest trans fat and it annoys me when I can't consume or purchase a product because I noticed there is a non-trivial amount of trans fat in it. In this way, banning trans fats is definitely a convenient solution for me and I don't see many downsides to supporting it.
But more generally, I think the argument might be "the cost benefits of transfats are very slight, and greatly outweigh the health risk, such that a very small fraction of consumers would choose transfats if they were well informed". Obviously this would need to be backed up with a cost-benefit analysis which tried to honestly estimate consumer preferences (rather than assign them preferences paternalistically), and I have do idea whether that has actually been done.