Who does belong there? Who should speak? What message? What is it that Amos has said that is so upsetting to those in authority?
Amos asserts that he speaks because the Lord has told him to. His speech has been full of radical social justice; e.g., oppressing the poor and crushing the needy (4:1); mistaking ceremony and offerings as a substitute for doing justice (5:21-27); and even more disturbing to us modern readers, living comfortable lives (6:4-8).
Do we get too comfortable in our lives to be able to hear God's call? Who gets our attention? Who does Amaziah think is his real boss? Who do we think is ours?