Shawn not have moved in because:
Shawn was making home-made porn
Shawn was a pothead
Shawn was cheating on her every second of their relationship, and she knew it (she cheated on him a few times, but he cheated on her constantly, he wasn't ready for monogamy)
She was still married to Clint and repeatedly refused to sign her divorce papers. Clint's girlfriend's mother (Patricia) eventually sued Billie to sign the papers. Gossip: her friends and family say she cried and cried over Clint, and begged him to return home. How could hubby forgive and forget and return home when you've moved your boyfriend into the bed you shared with your husband?
Before he moved in, Shawn physically threatened her and Hailey and David, and threatened her to the point that she called the police on him multiple times. She also called the police on him after he moved in.
She feared what would happen to David if David was left alone with Shawn.
She didn't trust Shawn.
She felt Shawn would rob her blind.
She had major problems in her life that she needed to deal with first, before taking on Shawn's problems.