The Mekhilta continues its rigorous legal analysis of who can be sold into servitude. Having established that a daughter cannot sell herself, a new question arises. Should a daughter be sold by the court for theft, the way a son can be?
The logical argument in favor seems strong. A son, whose father is not permitted to sell him, is nevertheless sold by the court for his theft when he cannot repay what he stole. A daughter, whose father is permitted to sell her, should logically face at least the same consequence. If the person with less parental sellability (the son) is sold for theft, the person with more parental sellability (the daughter) should certainly be sold for theft too.
The Mekhilta rejects this reasoning with another distinguishing characteristic. A son who steals and is sold by the court has another unique legal feature: if he refuses to leave his master's service at the end of his term, his ear is bored with an awl at the doorpost (Exodus 21:6). The ear-boring ceremony applies only to male servants. It is part of the same legal system that permits the court to sell a man for his theft.
A daughter's ear is never bored. She is not subject to the ear-boring ceremony under any circumstances. Since the court-sale-for-theft and the ear-boring belong to the same legal framework, and since a daughter is excluded from the latter, she is excluded from the former as well. The two provisions are linked. You cannot have one without the other.
This passage showcases the Mekhilta's method of legal distinction. What appears to be a simple logical extension is blocked by identifying a structural difference between the two categories. The son's legal profile is a package. Remove one element, and the entire framework no longer applies.