one of mcmurphy's surprises occurs when he finds out that the inmates are there by choice. They committed themselves and they remain there of their own volition. why did they make this choice?

Independence vs. Acquiescence

Throughout the novel, we consistently root for the inmates to find freedom, either through a mass escape or by overthrowing the regime and winning a new order in the institution. This is all subverted, however, when McMurphy discovers that he and Scanlon are the only two involuntarily committed inmates. The rest of the inmates are there by choice. They would rather be quiescent followers, surrendering themselves to institutional oppression, than independent in a society where they do not quite fit and may not be able to function. McMurphy sees emasculation as the prime reason for the choice to stay. The Nurse has found a way to mentally castrate each and every one of the inmates--including Rawlins, who commits suicide by physical emasculation. McMurphy may perceive that the best way to free the other men is to expose Nurse Ratched as flesh and blood rather than an inevitable oppressor--someone with her own flaws and pains. McMurphy attempts to work within the Nurse's system, trying to outmanipulate and outfox her with his various schemes. But ultimately, the only way to change the acquiescence of his fellow inmates is to lead by example. He feels presure to acquiesce and avoid pain, but he choose to follow his independent spirit, which explodes in brute force when he rips the Nurse's clothes open. This act prevents the rest of the inmates from ever seeing her as merely the robotic hand of authority. She has a body now, and they can no longer follow her blindly, understanding that she is just as mortal as they are. They are likely to continue choosing the institution to the outside world, but they will remain with a greater degree of independence than before.