Been there done that. It's very annoying indeed. I wish things were slightly less finicky in *NIX platforms as a rule. They need a serious de-crufting session (plan 9 looks very much less crufty).
I eventually just added it to the crontab of a user and that was that, but I'm so glad to finally know WHY.
This is such an arcane restriction and its really hard to debug since there are no errors anywhere.