It's a historical quirk of the industry. This stuff came around before Open Source Software and the OSI definition was ever a thing.

10BASE5 ethernet was an open standard from the IEEE. If you were implementing it, you were almost certainly an engineer at a hardware manufacturing company that made NICs or hubs or something. If it was $1,000 to purchase the standard, that's OK, your company buys that as the cost of entering the market. This stuff was well out of reach of amateurs at the time, anyway.

It wasn't like, say, DECnet, which began as a DEC project for use only in their own systems (but later did open up).

And then you have things like "The Open Group", which controls X11 and the Unix trademark. They are not particularly open by today's standards, but they were at the time.