I was thinking the planed was on a stop over in boston before continuing to somewhere else.
I dunno. Haven't been overseas in awhile. But AFAIK:
These flight were chosen because they were transcontinental flight with large fuel loads and they were at the beginning of the trip.
A flight ending in the US would be at minimal fuel.
Most International flights terminate at the first place they land in the US, for a myriad of reasons. They, generally, do not stop in NYC, refuel, and continue to Chicago, if that's what you were thinking.
Generally speaking, it's London to somewhere in the BOSNYWASH area, or say, Tokyo to LAX or SEATAC. The plane is unloaded and the passengers and cargo are unloaded and get on other planes for follow-on flights to their destinations. The long hauler is then checked over, searched, refueled, resupplied and sent somewhere else.
Generally speaking, the Transatlantic, Transpacific routes are served with dedicated airframes that ONLY go transoceanic. Their sister airframes, despite the fact that they are the same make-and-model airframes, only go transcontinental. Not sure I understand why this is true, but I know that was standard policy at Delta and United - the explanation was long, involved and esoteric.