Because they didn't want the victim to scream or whimper.
The police were responsible for setting fire to the homes of people who had books, as books were considered dangerous and so were the people who read them.
In that case, the police should arrest the person who had books and burn that person's house with everything inside it. This work should be done discreetly, without attracting attention, and with as little noise as possible.
For this reason, the victim's mouth was taped together to avoid making noise and attracting other people's attention and not being able to talk to the police and influence them not to do what they had to do.
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