Whoever told you that is lying to you. You are not legally allowed to personally memorize and recite copyrighted works all you want, any more than you're allowed to personally memorize, write down copyrighted works, and distribute them as much as you want.
All piracy is a process of computer-assisted remembering and reciting.
I can't go write and commercialize what I learnt directly, but I'm not breaking the law by quickly seeing how some book I didn't buy ends so I can talk about it at a party - and then everyone knows how it ends which might affect whether they want to buy said book and upset the author. But, tough shit, what I did was legal. I can even use the ending as one set of input from dozens of inspirations for my own book where the end result is transformative enough where the sources are unrecognizable. And if I had learnt about the endings from a dozen books without buying those books I didn't break any laws even though I am now commercializing something in being inspired by them all to make something new.