The GPL is likely the first, and probably the most employed CopyLeft software license.
The licenses covering most other FreeSoftware: such as Apache, Artistic, BSD, ..., PublicDomain, ..., Zope, etc. are not CopyLeft, and so are prone to exploitation by proprietary shops (Microsoft and Apple OperatingSystems contain much BSD code).
Even though the owners designated these resources as 'Free', they didn't say that a user may not 'hoard' the resource.
Hoarding may not seem to make sense for a 'virtual' thing such as a computer program - which can be copied for very little cost. But source code is more important as a means to production, than as a product itself.
When a playfully clever(1) mechanic looks at a machine, he usually finds problems to fix, or something that should be lubed or adjusted, but he may also have dreams to take this machine apart and mix it with parts of other machines.
When a hacker looks at code, he usually finds problems or has ideas about a better way.
If the worker modifies the device for the betterment of the community, he should be rewarded.
But if he uses IPR to stop the community from manufacturing a copy of this machine, and won't let them use the machine at cost, then we see the community divided through ArtificialScarcity.
(1) Richard Stallman defines the word 'hacker' as "playfully clever".
CopyLeft is the only problem IP Manoralists like Microsoft and Tivo (for GPLv3) have against FreeSoftware, because the resource cannot be locked closed as they wish, but must be kept available for all to use.