On most flights from Europe to the USA that I've flown, the in-flight DF was so "low key", that I had to get a FAs attention (on the 2 occasions when I wanted to purchase DF).
As I noted in a few posts back, DF out of ZRH, CDG and LHR were NO bargains--but when I checked DF shops at JFK, they were even worse.
One big PLUS for having DF in-flight, is that if you forgot (or had no time) to do airport DF, you still had your LAST CHANCE.

PS: As a general rule many European countries have "sales" twice a year--in the USA that restriction does NOT apply--I can understand why Europeans can find DF more of a bargain than Americans do.

PS(PS): At LHR T3, purchasing airport DF and carrying your (possibly heavy) item to say Gate 42 [many AA LHR-JFK flts. use this gate] is a LONG walk (with only 2 moving belts).