How do they get away with that ? Simple.
Don't believe that the campaign files with PC-Gen allows you to "know" the non-open books for free. I mean, you can learn things like there's a feat called Treetopper in the FRCS. Great. That don't explain you what the treetopper feat does. You do know its prerequisites; but that's all you have in the way of the game mechanic. If you absolutely want to use the treetopper feat in game, you'll have to invent your own game effect or buy the FRCS.
I could say the same thing for a prestige class from Song & Silence (OK, so the Temple Raider of Olidammara must worship Olidammara...) or a class from Star Wars (so, there are three Force classes: Jedi Guardian, Jedi Consular, and Force Adept...).
You'll have the BAB and save progressions, the skill list, skill points, and bonus feat. You'll have the name of the special abilities, but you won't have any description for them. How much good is it to know you have Supreme Cleave if you don't know how Supreme Cleave works ?
You know, I've seen MORE closed rule materials on Wizards' very own message boards than on PC Gen.
They can't control all the stuff typed-in by contributors, so sometimes you can find a prestige class for which all class features are explained lenghtly in the strings normally dedicated for the name of the feature (that is, instead of writing "Big Hullahoop +3", you'll find "Big Hullahoop +3: gives you a +3 profane bonus on all rebuke undead check made while performing a hullahoop move-equivalent action"). Such things are rare and usually get fixed as soon as noticed.