This is made more irritating by the fact that setting the User Agent to "iPad" on OS X has always yielded a perfectly functional HTML5 player.
That plugin must be closed source and must be ported to any platform that uses it by whoever wrote it (I think Adobe have one, and probably Microsoft too).
In some ways this is a step back from Flash.
A lot of the stuff they show is owned by other companies (films, sports, etc.). They also commission a lot of stuff from outside production companies who want to be able to commercialise it in other ways afterwards (e.g. sell it on to Netflix afterwards, show on repeat channels, etc.).
Other reasons include not owning the rights to things within the show, most notably music.
Eg Top Gear got away with using music from many famous films in almost every show, I thought this is how they were allowed to do that. And their "documentary" status is questionable.
For example, Apple's is called FairPlay. It's automatically embedded in every iOS device, Apple TV, Apple Watch, and Mac. It is able to decrypt the content streamed from approved providers.
Of course, it is closed source, but that does not make it insecure or dangerous. The vast majority of your Mac is closed source and has an awful lot more privileges than FairPlay.
Encyrypted content is trivial to record and redistribute for reasons that Cory Doctorow spelled out to tech companies a decade ago. That those companies are still pretending otherwise is leading to them perverting the operation of network and systems software.