It is a Apple developer conference for software developers that care about making beautiful applications that take full advantage of native experiences on Apple platforms.
Any developer on this community can watch the video.
I can still be an outsider and be genuinely interested in what their platform has to offer and, maybe, later decide that I want to invest in their platform. I become a better developer, Apple gains a new developer, everyone wins.
Forcing people to use macOS to view recorded sessions or events goes against that.
However, the default experience is a little box that says "Streaming is available in Safari and the WWDC app". Now I have to go out of my way to learn that they stream via HLS, obtain the stream's URL and feed it to VLC. †
Compare this with Microsoft's and Google's videos (available through Channel9 and YouTube, respectively) that are accessible "virtually" from every operating system and device.
If your goal is to attract new developers to the platform, maybe adopting a more widespread industry standard (such as DASH, which all other browsers implement) is probably the way to go, IMO.
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† By the way, the box also breaks the "Copy Video Location" menu item, so I have to open the inspector or install an extension to find out what the real URL is.
It does seem like a lost opportunity to do some outreach though. It's not hard to provide streams both in HLS and DASH, and streaming the developer conference to everyone could surely attract new developers to their platform (and not only developers, the same thing applies to presentations of new devices and consumer software).
To me it really looks like a statement against people without macs; Apple doesn't care about them, not even in its own interest of making money. They won't speak to you as long as you don't have an iDevice, full stop.
All computer ecosystems up to the late 90's were like that, the only thing special about Apple is that they are the surviving ones from those days.
It were IBM "mistakes" that made the PC different from all other computer eco-systems, however the current trends of commodity hardware and race to bottom prices are making PC OEMs go back to the 80-90's full integration of hardware/software culture.
Any developer on this community can watch the video.