Not sure if I missed any earlier thread, if so please redirect me, but I havent seen anything about a new "stopover fee" that I got charged recently.

Wanted to buy a round trip ticket to USA from Europa via LHR to JFK and back from MIA, with an internal domestic leg that usually I asked to be ticketed in the same trip as one. However, I was told if I dont continue my trip within 24h in NYC as I planned to spend some days there, the "stopover fee"!! is EUR600 for premium cabins. 200 for economy.

What the heck is this? Now I had to buy my domestic ticket separately. What is this good for?