California is an at-will employement state.[1] Riot can terminate those employees for any reason not protected under federal or state law, such as being a member of a protected class. Being a gigantic dick does not grant protected status. The only liability Riot has here is for their unemployment, unless they can show cause for termination, which is probably more of a gray area.

[edit: forgot the link]

[1]http://business.ca.gov/StartaBusiness/AdministeringEmployees...