Basically...Jimmy Swaggart had to pay CA a large amount in sales tax due to the sale of religious lit. He paid it, then appealed and lost. It became law that the sale of religious lit was subject to sales tax...and that is why the WT changed its arrangement.
Let me find you a link with the precise details....
http://www.bibletopics.com/biblestudy/84.htm
Go there- it gives the whole picture...