why the eved is punished only after the fact

Our parsha teaches us that if an eved ivri chooses to remain with his master instead of seizing the opportunity to go free at the end of his servitude, his ear is pierced as a punishment and he remains enslaved until yoveil.  Why pierce the ear?  Rashi quotes from Chazal that the ear that heard on Sinai that ‘Bnei Yisrael are servants [only] to me [G-d]’ and ignored that message and took a human master deserves to be punished with piercing.    

Everyone asks: if that’s the reason for piercing the ear, shouldn’t it be done when the individual first gets himself sold into slavery?  Why wait until the sentence is over and he chooses to opt for an extension?

R’ Shmuel Ya’akov Rubenstein in his sefer She’eiris Menachem answers that when a slave is first sold, he may not realize what he is getting himself into.  The full force of what slavery means may hit a person only after he experiences it, after he tastes what life is like under else’s thumb, and after he has suffered a bit.  If after all that the person still prefers slavery to living as a free man, then the Torah says this person deserves the punishment of piercing.  

The Shem m’Shmuel explains that the eved ivri suffers from delusional thinking.  He comes to his master and claims that he prefers to remain enslaved because he loves his wife and his children, “ahavti es ishti v’es banay…”  He does not realize that “his” wife and “his” children are merely chattel, property of his master.  He entered slavery perhaps not realizing fully what he was getting into, and after spending time as slave he is no wiser to his circumstance than he was before. 

A person sometimes cannot be blamed for making a wrong decision.  The full import of choices are sometimes realized and felt only after the fact.  But a person can be blamed for ignoring reality and not learning from his circumstances.