Yes, it's not a bug in Vixie Cron (which I think BSD cron may use) but in the Linux distros (who are responsible for adding run-parts).

.py, .pl, .sh and (as people realize they'are actually writing bash scripts) .bash are standards people actually use. Having them break in what amounts to an include dir is bad mojo.

I can't speak for other BSDs, but FreeBSD's cron is still derived from Vixie cron.

OpenBSD uses Vixie Cron as well.