I seem to remember being told that the back of the net was part of the field of play, too.That's right.
The law makes sense, otherwise a player could simply step off the pitch to make an attacker offside.
I suppose you could introduce some idea of whether they did it deliberately or not, but that would open a whole new can of worms. In any event,
Can't remember it ever being an issue before in a goal.
A forward standing there could be given offside and a defender there could be regarded as playing him onside.
Once played in a game where the Ref gave offside for this, cue mayhem.