They were all proving to the emperor that even though they married foreign ladies, they harbored no intention of treason.
The emperor didn't mind this. In the future, their descendants, while ineligible for the throne or military command, could still hold various official positions.
The royal bloodline demanded purity. Once a royal descendant married a foreign princess, they lost the qualifications to inherit the throne.
This was a common occurrence in history. Even if an emperor married a foreign princess, she would only be granted the title of concubine. Any outstanding sons born of such unions wouldn't ascend to the throne.
The old emperor cared about tradition, and the officials understood it. They quickly drew a line to make the emperor feel at ease. It had its benefits, at least to ensure the emperor's trust.
