I have noticed this in the past and fully agree. The author could have even gone a step further to point out all the cases of people who are labeled immediately as terrorists. Generally, if you are Muslim and attack people, you are immediately labeled as a terrorist. However, if a white Christian attacks people because they are too liberal/ruining your country/whatever, it was often just some guy that was "crazy." There seems to be a disconnect in the general willingness to label such persons as something other than crazy. There are perhaps some nuiances that need to be discussed, but I do think we need to work on how we address these topics in the U.S.
I don't even think most of these guys are crazy. They could be making rational decisions based on faulty evidence. Storm roof thought he would be a hero (and is to some extreme racists), Anders Brevik, the guy who flew his plane into the IRS building, Christopher dorner, the list goes on. These people were not mentally ill. They made a conscious decision and carried it out. They felt (in some of the manifestos) that there was no other choice. I mean some are mentally il some arent, but we have problems underlying our society that go ads this behavior in a way.