Your comment is used by Birchers to discredit rational and legitimate criticisms.  The JBS (as an organization) is not anti-semitic but, regrettably, it has had problems with anti-semites who joined and, in some instances, the JBS has repeated anti-semitic themes or cited anti-semitic sources in publications it recommends. There also is the matter of comments made by Robert Welch concerning Israel and zionism which are troubling.  AND there was the controversy over anti-semitic remarks made by former JBS President John McManus.

However, I doubt that several hundred Jews would join any organization that actually was anti-semitic.

One has to remember that all large organizations face the problem of what I call "boarding parties", i.e. noxious individuals who attempt to hijack or manipulate the organization into adopting their personal viewpoints.  The problem I have with Birchers is that they refuse to acknowledge their own history--even when evidence originates from other Birchers!