Shibboleths exist to exclude people outside the subculture stereotype - and a good way of doing that is to be offensive. Some subcultures desire to remain as subcultures.

Part of why shibboleths work is because we as a species tend to stop engaging rationally the moment we feel attacked - we switch to being defensive and actually entrench our own values further.

The 4chan shibboleth attacks everyone. No matter who you are or how you identify you're going to be debased and mocked openly - that's kind of the point.