I am really only dicking about. It's about jumping technique.
Players going for a header are going for the ball in a different way, they don't need to use a knee up as it would actually make it harder to jump effectively for the ball and generate power.
Keepers need to bring their knee up when jumping for a high ball not for protection, but because when they are jumping vertically for a the ball it's a natural body movement and helps with the power in the jump. You can't jump effectively for a high ball straight up 8 feet in the air with both legs straight down. Raising the knee also means if someone does jump into you, you're less likely to be knocked off balance and lose control of the ball in the air.
What Hennessy did was different, even if he got the ball, he probably should still be sent off.