One possible cause might be that someone created a sample database using db2sampl. The sample database contains both of these artefacts:
]$ db2 -x "select tabschema, tabname, type, clustered
where type = 'S' or clustered = 'Y'"
DB2INST1 ADEFUSR S -
DB2INST1 EMPMDC T Y
It appears as if something in the instance is flipped during create of sample database, dropping the sample database does not help, the violation persists.
As @Chris Aldrich points out in the comment,
db2licm -x does a reset of the license report:
DB2 Workgroup Server Edition In compliance
Out of curiosity I created 1 MQT to see if the violation re-appears. So far it has not. It's now more than 12 hours since I created the MQT and I'm still in compliance.
I'm still puzzled by how the licence compliance mechanism works, but I'll add another question about that.