Back then if you wanted to use C or UNIX you had to license it from Bell Labs. If you wanted BASIC you had to license it from Dartmouth, if you wanted COBOL you had to license it from The navy and Grace Hopper, etc.
Over times things have changed and free and open source versions of those languages got written. But try to make a free and open source version of the Intel Mac BIOS, try to make a clone of MacOSX, try to make a version of XCode that runs in Linux and submits apps to the Apple App Store, etc. You need an Intel Mac for that and Apple's MacOSX and XCode. So not everything is free and open source or has an alternative to it.
For example Microsoft made Windows that basically stole the Mac GUI IP without permission. Windows sold so well that Apple started to have financial problems. In that case it was taking money away from Apple.
Also, while you had to get a UNIX license from AT&T, they didn't make any money from it (well, they weren't allowed to) and at the time, it came with full sources. Yes, the term FOSS didn't exist yet but people shared source code anyway, at least in the academic/research environments where UNIX was popular.
You're right in that things were much worse in the commercial office and home computer market.
What IP holders? Those of other FORTRAN implementations?
So if I'm writing a C compiler I'm "taking money away" from the developers of existing compilers? Like, I'm doing something despicable, as apparently, that money belongs to them?