Why did Silas take away some of the memories of Scarlett and her mother?

After Scarlett witnessed the Jacks being attacked in the graveyard, Silas likely knew that both her trauma and the risk of her speaking about the event was too great a danger to Bod. He changes her mother's memories to help convince her to move—which means that Scarlett won't be close enough to visit the graveyard anymore.
Bod finds out that Mr. Frost, the man dating Scarlett's mother, is the one who killed his family. He and his group, the Jacks, believed that Bod would be the end of them. They attack him and Scarlett in the graveyard and Bod deals with them in multiple ways, including having one break an ankle and fall in a pit. Another is pulled through the ghoul-gate.
After Scarlett sees the last man taken by the Sleer, she panics and says that humans don't do what Bod did. Since she can't see the Sleer, all she sees is Mr. Frost hanging in midair trying to get away from something. It looks like he is being "forced through the wall, pulled into the rock, was being swallowed up by it."
Silas arrives and says that he's going to decide what she should remember and what she should forget. Bod argues, but Scarlett says she only wants to go home. Silas says forgetting is likely safer for everyone but most definitely safer for Scarlett. Then he takes her home, explains things to her mother—that her mother can't quite remember later—and Scarlett and her mother move back to Scotland.
When Silas returns to the graveyard, he tells Bod that people want to forget the impossible because it makes their worlds safer.