Doesn't "we can't interfere with a religious practice" plus "we can't determine what is a legitimate religious practice" mean anyone can claim anything to be a religious practice? "Oh, I'm sorry, I'm engaged in the blessed sacrament of the mugging."
This is where it's worth remembering that humans are in the justice system by design. While the line is intrinsically fuzzy, one can at least agree that at one extreme a marijuana user coming up for charges who suddenly claims he has a marijuana-based religion in an attempt to get out of it isn't going to get very far. Judges are not required to be as stupid as computers interpreting the law.
On the one hand, judges are still part of the government. On the other, the first amendment says "Congress shall make no law regarding the establishment of religion", not that "judges shall make no rulings".
