Why Sorter is an Active transformation ?

The Sorter transformation is almost always changes the order of records - by delivering the records sorted by the sort keys. 

There are some cases when a Sorter doesn't sort the data because of input records are already sorted. But still the Sorter MAY change the order of records, so it's always considered as an Active transformation.

The DISTINCT feature is just an add-on. 

In most cases it will decrease the number of records (by removing duplicate records), but that doesn't impact the fact that the Sorter is an active transformation: the mere chance of changing the order of records is sufficient for the classification as an "Active transformation".