The labeling requirement is theoretically protecting consumers from being ripped off by paying for water when they were expecting fat, but the manufacturers have convinced consumers this is actually a good thing ("reduced fat"). The real reason all spreads in the UK now have <80% fat is most likely cost reduction.
Will everybody on HN please stop assuming the "law" is as it is in their local jurisdiction, and why posting a technical industry article on different fats is meant to convince me I'm wrong in establishing that in the UK (the country in which this thread's food is considered), there are specific terms used for specific products, I can't fathom.
"Products in the form of a solid, malleable emulsion, principally of the water-in-oil type, derived from solid and/or liquid vegetable and/or animal fats suitable for human consumption, with a milk-fat content of not more than 3 % or the fat content.
Margarine
The product obtained from vegetable and/or animal fats with a fat content of not less than 80 % but less than 90 %."
No mention of partial hydrogenation or trans-fats. The only reason UK spreads are not "margarine" is because they do no contain enough fat.
The labeling requirement is theoretically protecting consumers from being ripped off by paying for water when they were expecting fat, but the manufacturers have convinced consumers this is actually a good thing ("reduced fat"). The real reason all spreads in the UK now have <80% fat is most likely cost reduction.