HEVC is provided by the official, licensed h265 standard. The open source ~HEVC-compliant codec library is x265 created by VideoLAN but was apparently not an option for Microsoft.

It's not about the code. The open source implementations are also subject to patent laws, they just ignore them and put the responsibility on the user. And users don't know/care about it so in the end they get playback for free.

That is why some distributions (RHEL derivatives, for example) do not ship support for many codecs out of the box and they make you jump to (admittedly simple) hoops to get it working.

x265 is an encoder, not a decoder. Also, being open source doesn't matter here: an open source library, even with a patent grant, doesn't give you a license to someone else's patents.

VideoLAN wants you to pay the royalty for x265 and you'll get sued by patent pools if you use it in a company (and are big enough).