Preserving freedoms downstream
Copyleft addresses what happens to permissions after a recipient modifies and redistributes material. The GNU GPL ties its grants to conditions preserving specified rights for later recipients.
It relies on copyright permission rather than declaring copyright absent. MIT-style licensing retains notices without imposing the GPL’s same-license framework on modified works.
The covered work sets the boundary
GPLv3 distinguishes a work based on the program from an aggregate containing independent works. Sharing one distribution medium does not by itself bring every independent program under the GPL.
CC BY-SA supplies a different example: shared adaptations follow the specified same or compatible licensing arrangements. Merely appearing together does not settle whether material is an adaptation.
Commercial use can remain permitted
Preserving downstream freedoms does not prohibit charging for copies: both GPL and CC BY-SA allow commercial activity within their terms.
Copyleft is not the name of one universal license. Versions, covered material and the action performed determine requirements concerning source, notices, attribution and compatibility.