Some US carriers chose to avoid based on FAA guidance; some HKG did so on their own threat assessment (which is good to do and make prudent decisions based on--but at best most are educated guesses--many more warning areas come up than shots are ever taken).
The mid FLs clear most IR and AAA threats but are well within engagement zones for radar missiles. Soviet systems of all kinds are widely exported--many to terrorist states like Iran as well as North Korea and many friendly nations too--including India (the SA-11/17 is operable in Finland among many other places). China has several exported soviet-style and copied (and capable) missiles. If you're going into Inchon, Moscow, China, anywhere in the mid east, etc. you're transiting multiple engagement zones--often between nations who don't like each other. As well as naval forces operating over open water from all nations. Imagine the folks with the same situational awareness (or less) of ATC going into PVG having their finger on the trigger and you might understand what one is dealing with transiting the mainland. FL300-400 for a non-maneuvering or limited maneuvering target is well within the engagement envelope of almost any radar guided SAM.
Most commercial operators are currently operating in all sorts of missile engagement zones--both by people who know what they are doing and are tied in to a coordinated air defense system and those not. Add to this during military exercises active airborne jammers may be present which could be intentionally or inadvertently taking out acquisition radars (which can provide ID on non-hostile tracks as well as hostiles to the individual SAM batteries).
As far as in THIS case, it was largely a matter of bad luck (the actual shoot down may have occurred outside the advisory area)--and it's a mobile SAM system anyhow--likely on "lease" to surrogates in the Ukrane (obviously they've been evicted). A group operating semi-autonomously (i.e. likely not tied in to a larger air defense system) took a bad shot. Somewhat capable missile in the hands of someone either not able (or not wanting) to make positive ID.
Ironically (although Putin will do nothing to change his position) the area might now be one of the safest areas to transit now. Given soviet justice I would NOT want to be the guy who pulled the trigger on this one; while Putin will continue to exploit and press his position it will be seen as a black eye and the perps will not be having a good life methinks (if a life at all).