Why do celebrities get out of jail early?

Lindsay Lohan for example should be serving a 90 day sentence. They said she'll be in, at the most, two weeks. They let Nicole Richie out of jail after, what was it 82 hours, because of overcrowding? She drove the wrong way down the highway, drunk! Most of us here would be in jail for a while! Same for Paris Hilton. She got out after overcrowding! Why do they do this? Why do they get special treatment?