If auto_snapshot is set to true in cassandra.yaml, each time you restart Cassandra, a snapshot is created for system.compactions_in_progress as the table is truncated at cassandra start.
However as datas in this table are temporary, Cassandra should not create snapshot for this table (or maybe even for system.* tables). This will be coherent with the fact that "nodetool listsnapshots" doesn't even list this table.
$ nodetool listsnapshots | grep compactions
$ ls -lh system/compactions_in_progress-55080ab05d9c388690a4acb25fe1f77b/snapshots/
drwxr-xr-x 2 cassandra cassandra 4.0K Nov 30 13:12 1448885530280-compactions_in_progress
drwxr-xr-x 2 cassandra cassandra 4.0K Dec 7 15:36 1449498977181-compactions_in_progress
drwxr-xr-x 2 cassandra cassandra 4.0K Dec 14 18:20 1450113621506-compactions_in_progress
drwxr-xr-x 2 cassandra cassandra 4.0K Jan 4 12:53 1451908396364-compactions_in_progress