Perhaps we are somewhat uncomfortable with this reality especially if it approaches nepotism, but we also respect that this is part of being a family and would be more disturbed by a parent that does not help. Merit marks the individual. When we evaluate on merit, we focus on the attributes and abilities of the person. Discrimination is deemed to occur when factors that are not tied to these personal attributes and abilities, specifically factors that connect this person to a group, are applied in situations when the individual alone should be considered. Inversely, when we do accept groupings not tied to merit, we are, in fact, placing group factors above the individual.
Citizenship is based on birth.
Citizenship by birth places the concept of country above the individual. When we see the kohain , we are not to see the individual but the value that the kohain represents as part of this group. They are a symbol of our collective connection to G-d. As a chalul , this son loses his status as a kohain even though his father is a kohain and he has done nothing wrong. The father who actually transgressed, though, still remains a kohain .