Westboro Baptist Church has been classified as a hate group by Anti-Defamation League and Southern Poverty Law Center; however, they continue to be tax exempt despite not meeting the IRS guidelines. According to the plain language of IRC 501(c), the Westboro Baptist Church should not qualify as a public charity because it clearly does not meet the requirements. The group is not organized and operated exclusively for exempt purposes. The actions of this group in picketing funerals and causing pain to citizens weeping for lost loved ones do not advance religion. Rather, the actions inspire hatred and anger.
More info here about tax law: http://www.irs.gov/pub/irs-pdf/p1828.pdf
More information about the Anonymous petition: http://www.dolmanlaw.com/westboro-baptist-church-tax-exempt-2/