Skills in practice: "what about."
In this case, "what about" is a 56 year old booster launched before Kessler did his calculations, before we had a clue about space debris, before there was an ISS to intentionally involve in a orbital path of debris purposely created and long after we knew better.
Falcon upper stages left behind now mostly in service of the orbital kitty video delivery system? That's a legitimate problem, but not one the authors of "what about" worry over, based on their own seemingly pointless action. It's worth noting also that upper stages made by SpaceX passivate as part of their normal procedure, won't fragment unless struck. Unlike the Fregat still used by the "what about" authors which routinely fragments after use, and the continuing problem of SOZ ullage motors which continue to be deployed and continue to routinely fragment due to "we don't care."
Indeed, the puzzle answer for _why_ the Russians did this might lie in work also done decades ago, the exploration of geopolitical posturing with nuclear weapons by Herman Kahn and others. Sending a message to the effect of "we're crazy" can have a strategic purpose.
But the "what about" authors can depend on the reliability of ignorance. That fog can be penetrated in this case by visiting ARES' https://orbitaldebris.jsc.nasa.gov/quarterly-news/
[I think I've got my spacecraft mixed up here; the Pegasus stage in question is probably the later Northrop-Grumann variety. But: passivates, isn't intentionally fragrmented in a dangerous manner.]