The Duty Free is provided by an outside vendor. I do not know who the vendor is but they pack the carts at the catering kitchen. The kitchen places the locked carts on board the flight and the FA's sell you the items.

I can understand the concern of the FA not wanting to use garage bags for items purchased since the possibility of running out of garage bags is not something they wish to have. All garage on international flights are treated as foreign matter and must be disposed on in a totally different way (it's cooked in a boiler to halt the spread of disease/foreign bugs and the FDA is very serious about this issue) unlike domestic garage which is just tossed into a dumpster.