That’s part of what I picked as a teaser:

Start with what the GPL actually prohibits. It does not prohibit keeping source code private. It imposes no constraint on privately modifying GPL software and using it yourself. The GPL’s conditions are triggered only by distribution. If you distribute modified code, or offer it as a networked service, you must make the source available under the same terms. This is not a restriction on sharing. It is a condition placed on sharing: if you share, you must share in kind. The requirement that improvements be returned to the commons is not a mechanism that suppresses sharing. It is a mechanism that makes sharing recursive and self-reinforcing. The claim that imposing contribution obligations on users of a commons undermines sharing culture does not hold together logically. The contrast with the MIT license clarifies the point. Under MIT, anyone may take code, improve it, and close it off into a proprietary product. You can receive from the commons without giving back. If Ronacher calls this structure “more share-friendly,” he is using a concept of sharing with a specific directionality built in: sharing flows toward whoever has more capital and more engineers to take advantage of it. The historical record bears this out. In the 1990s, companies routinely absorbed GPL code into proprietary products—not because they had chosen permissive licenses, but because copyleft enforcement was slack. The strengthening of copyleft mechanisms closed that gap. For individual developers and small projects without the resources to compete on anything but reciprocity, copyleft was what made the exchange approximately fair. The creator of Flask knows this distinction. If he elides it anyway, the argument is not naïve—it is convenient.