Probably because they know, that unlike the panthers, the DOJ would be all over their cases. I do not like the klan but I an so fed up with this crap my eyeballs are melting. :>)
Beck made a comparison to the white league which is a far better comparison for the black panthers.
From Wiki
>>The White League was a white paramilitary group started in 1874 that operated to turn Republicans out of office and intimidate freedmen from voting and political organizing. Its first chapter in Grant Parish, Louisiana was made up of many of the Confederate veterans who had participated in the Colfax Massacre in April 1873. Chapters were soon founded in other areas of the state and in New Orleans. During the later years of Reconstruction, it was one of the paramilitary groups described as “the military arm of the Democratic Party.” Through violence and intimidation, its members reduced Republican voting and contributed to the Democrats’ taking over control of the Louisiana Legislature in 1876.<<
http://en.wikipedia.org/wiki/White_League
As always, wiki is just a starting place so its wise to look further.