No one runs pure zone, but things like having a big sit back to corral pick and rolls, or refusing to guard a non-shooter on the perimeter to operate as a floating defender and other forms of unconnected defenders are how defensive schemes incorporate zone concepts now that they couldn't do before when you had to be within three feet of an offensive player and you could only double-team on the ball (which is what I meant by "illegal defense"--defensive three-second is a separate violation in which you can't spend more than three seconds in the paint without actively guarding an opposing player).
which is what I meant by "illegal defense"--defensive three-second is a separate violation in which you can't spend more than three seconds in the paint without actively guarding an opposing player).