Please mind that when doing upgrades, a regular file would probably not get updated, while using symlink will assure you that the newest timezone file will always be used.
Please mind that I didn't invent this. I was only reporting what Debian's standard package does. As far as I can follow tzdata's postinst script it re-copies the new zoneinfo file to /etc.
Also, if you need /usr to be available when the zoneinfo access is crucial, and it is not already mounted, you have bigger problems.
That's why a regular file on / avoids this.
