But is that illegal? Or is that violation of copyright?

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Who cares if it's a "violation of copyright" ? Copyright has become a set of overbearing corrupt laws bought and paid for by corporations. Sure there are some vestigial bits in there that an individual can kind of use if they do everything exactly right, but the overall dynamic is skewed very hard in favor of corporate control.

Apart from the blatant abuse of the legal system to computationally disenfranchise individuals via the DMCA et al, the main issue here is using centralized websites as watering hole for authoritative development/distribution. It would be understandable if Github were merely a mirror for exposure etc, but that should be the extent of it.

I think the fact that they publish the site in a readable state first and adding the paywall after some time (after getting indexed / shared a bit) and services like archive.is just provide a copy of the original state makes it debatable if this is a circumvention.

If I publish a project on GitHub with a BSD license and after some time switch to a proprietary license the old code would still be licensed under BSD, right?

The content published on the site in the open was never licensed to you though.

Lets say I make a movie and do a free screening of it in a public park to get people talking about it. That doesn't mean that it's ok for someone to record that screening and for people later to go looking for that recording rather than paying to watch the movie.

And being sent to a computer and stored in memory to render it to you doesn't change that either. That's necessary for the work to be displayed to you over the Internet and the law understands that (see: "What Color Are Your Bits?")

I like your movie screening example.

Let's say you want to watch a movie about a certain topic. You ask a couple of friends if they can recommend one. They respond by telling you about this free movie screening at the local park - the movie is exactly about the thing you're interested in. You go to the park. Five minutes into the movie they start asking for money or you have to leave.

- I think your friends might not have recommended the movie if it wasn't free - you might not have gone to the movie if it wasn't free - nothing on the outside of the movie screening area indicated that they switched to a paid screening

I feel like there is a difference between a free screening to get some publicity and a simple bait and switch.