Very strange sentence in Rule 8 Section 1 Article 8

It is a foul for illegal touching if a forward pass (legal or illegal)
thrown from behind the line of scrimmage:
a. is first touched intentionally or is caught by an originally ineligible offensive player; or
b. first touches or is caught by an eligible receiver who has gone out of bounds, either of his own volition or by being legally forced out of bounds, and has re-established himself inbounds.
Notes:
(1) If a forward pass (legal or illegal) is caught by an ineligible offensive player, the ball remains alive. (!!!!!?????)

WHAT? How can it be? "If a forward pass (legal or illegal) is caught by an ineligible offensive player, the ball remains alive."
Please explain this note