If you arrive in the US they explicitly tell you to put duty purchases in the checked luggage on arrival if you are flying onward. The TSA doesn't care two hoots about the EU sealed bag.
The same goes for DF goods bought on board LH. Some flights have the EU DF sealed bag available, so if you buy liquids on board you have demand the bag in order to preserve the goods in transit at FRA or MUC. So the OP could buy something on board and ask for the bag to seal that and the other DF liquids