[Adam]

I disagree that intent is required here for the violation.
If A2 steps clearly OOB, you have no idea whether he knows he's out or not.
And, frankly, whether he steps on the line or a full foot OOB, his intent is the same. If you think he's intending to skirt around the player by stepping on the line, are you going to call this a violation.
Secondly, lets say the defender (in the OP) purposefully puts his foot on the line to close that gap. Are you going to call the violation?

My point is that if you define the playing court as completely in bounds for purposes of a stationary player being entitled to a spot, then you have to call this violation.