BS2000 is the result of corporate espionage. Fujitsu, Hitachi (VOS3) and even Japanese government officials were in on it. They stole the source code to IBM's MVS operating system.
https://fas.org/irp/congress/1989_cr/h890712-japan.htm
And yes, the BS2000 uses the /360 ISA, but so did a few other imitators of IBM. And copying an ISA is widespread, and not restricted to the /360.
What you say is true of Fujtisu MSP. But there was never any stolen IBM code in BS2000. BS2000 was originally developed by RCA (as TSOS) without using any IBM code, and then Siemens purchased TSOS from RCA–and used it as the basis for BS2000–and then Siemens in turn sold BS2000 to Fujitsu.
Fujitsu had motivation to steal IBM code for MSP since MSP was trying to be MVS-compatible. RCA/Siemens/Fujitsu never had the same motivation for BS2000 since BS2000 was never aimed at MVS compatibility.
Software as busy is mainly bill gates idea. Control the hardware (like ps2 ...) you have the castle is the mentality of yesterday year. Or meter the punch card (the first anti-trust case against ibm?).
In the beginning (1960s), IBM released their mainframe operating systems into the public domain. (At the time, US copyright law required an explicit copyright notice, and so IBM relinquished their copyright by publishing the software without any copyright notice attached.) So they were not copyrighted and anyone was free to use them, both then and today.
In the 1970s, IBM began to increasingly copyright add-on components, but left the core OS public domain. However, a lot of new features went into those copyrighted add-ons (MVS/SE, MVS/SP, ACF/TCAM, ACF/VTAM, TSO/E, SPF, etc), so if you just stuck to the public domain core OS you were running something a lot more limited than what you got with all of IBM's copyrighted add-ons.
Finally, starting in the 1980s, IBM started copyrighting the core OS as well. The big change was the introduction of MVS/XA in 1983 – MVS/XA supported 31-bit addressing (as opposed to the 24-bit addressing of previous versions). MVS/XA was fully copyrighted, both core and extensions.
So, yes, you can run old public domain OS versions legally, but not current versions. For example, the newest version of MVS you can run is MVS 3.8J at 8505 maintenance level. That has bug fixes through to 1985, but in terms of actual features is stuck somewhere in the early-to-mid 1970s. (The story for VM/CMS and DOS/360 is similar.)