That's a light spec for a subset of the interface and behavior of the crontab command line frontend, not a standard for cron in general. It's not really relevant to a discussion about how distros set up their cron defaults for per-user access.
Thanks for sharing it though. I wasn't aware it was in the posix spec and it explains why pretty much every implementation supports .allow/.deny files even when most already implement better access control mechanisms.
Per Gentoo's wiki, both fcron and cronie have their own (different!) ways of whitelisting non-root users to run cronjobs.