"Go to Bethel, and sin; To Gilgal, and sin more. Bring your sacrifices every morning, Your tithes every three days,
"And proclaim freewill offerings, and publish them ..." The violation here is two-fold.  Freewill-offerings were not supposed to be motivated by any specific regulation, but the people of Amos' day had "proclaimed" freewill-offerings, that is, exhorting the people to give them.  They were supposed to be absolutely spontaneous; and these proclamations were a violation of that intention.  Furthermore, as an added incentive, they were "publishing" the freewill-offerings, that is, they were publishing the names of the donors!  This, of course, was a good fund-raising device, but it was contrary to God's will.