Nobody explicitly decided to enable the logging by default. It was caused by an uninitialized variable. Uninitialized variables result in undefined behavior, but can often go unnoticed in practice if the code around them results in the value in memory (or register) for that variable usually being a certain value. In this case, it happened to result in logging being disabled by default in most situations. But using jemalloc for the process rather than the standard malloc implementation was enough to cause the opposite result, apparently due to jemalloc poisoning malloc()ed regions (which overwrote the "usual" value).
And in case anyone thinks that sheer developer machismo (or 'rigor' or whatever) can keep you out of harm's way here, then I suggest reading John Carmack's excellent writeups on the horrors found by static code analysis.[1][2] Even the best programmers out there will get caught by any number of language traps.
[1] Sadly/happily, still available via archive.org. CSS seems to be busted here, so scroll to the bottom for Carmack's article:
