Indeed. First off, Fa'a'ā International Airport is relatively small. Unsecured elevator movements while parked in a high hydro-carbon contamination area, (jet blast), may, over time contribute to several problems, one of which is premature control strand failure.
Why the DHC-6 aircrew may or may not have routinely installed a control lock is undetermined. The direction the DHC-6 aircraft were routinely parked on the apron is also subject to investigation.
Most pilots I've flown with, (and taught), know enough to park upwind and install a control lock.
As an aside, in the late 1970's and early 1980's, the BN-2A Islanders were marshalled well away from the big jets at Pape'ete's Faa airport.