I found this-
Butter has virtually no lactose (and thus it is legal). Cultured butter is
regular butter with a bacteria culture added to it to give it taste. This
culture will eat up any lactose that remains in regular butter. So cultured
butter has less lactose than regular butter.
And this-
As for butter being OK and cream not. Yogurt made out of cream is OK as is legal sour cream. There is residual lactose in cream - I'd say about 5-10% and when you churn it, it separates out with a little water (buttermilk). In fact there is probably about less than 1% lactose in butter but it seems to be OK - better than margarine. Some dairies makes cultured butter where even the tiny residual amounts of lactose are changed to no lactose.
I also found this-
The 2% rule: There is a rule that allows some ingredients that constitute less than 2% of the total weight or volume unnecessary to report on the ingredient list. This can include sugar and other illegal ingredients, since these do not fall under the list of usual allergens.