NFHS Rule 12-3-2
"A player shall not, in any manner, charge the goalkeeper in the penalty area unless the goalkeeper is obstructing the player or dribbling the ball with the feet."
There's a bit more than that, but the way it was interpreted when I reffed in KY and now that I ref here in FL is that if the keeper is going for the ball to make a save, he/she can not be touched by the opposing team. Any contact is a foul - up to the ref to decide whether to (a) play on (b) just call a foul or © call a foul and issue a yellow or red card depending on the severity of the contact
You can still call a foul on the keeper if, in the ref's opinion, the keeper went out to play the person instead of the ball. Keeper's don't have carte blanche to just go charging through their opponents, or to come out and kick/pull/push/jump into anyone to get to the ball. They're just more protected under NFHS when they are making a legitimate play on the ball in a save situation (because they will tend to be in a potentially compromising/dangerous position when doing so).
I'm an ex-keeper so I like the extra protection afforded by the NFHS rule; I would certainly understand why attacking players would not like it as much.