Legal guarding position question

[sj]

A5 is dribbling up the court near the sideline. Defender B21 establishes legal guarding position with both feet in-bounds. Just prior to contact B21 has moved to maintain legal guarding position and has one foot on the sideline when A5 runs into B21 knocking him over.

Based on the wording in 4-23-2-a and 4-23-3-a, as well as the comment on page 70, this is a charge. Is that understanding correct? And seeing they clarified this rule a couple of years ago does anybody have any idea why they still allow the defender to have a foot out of bounds while he was just maintaining legal guarding position?