Believe it or not, religious restrictions can be oh so much more powerful than laws. Mentally, i mean.
The problem with religious restrictions, at least in the case of Christianity, is the punishment. You sin, you are sent to hell. There's no variability in punishment.
Here's an example: if the punishment for every crime was death, there would be no reason to perform more severe crimes once you've performed a small on. This means that someone who, say, litters, assuming littering is a crime, then they would be punished by death if caught. Now, if no one seems them or finds out, no worries, but say someone sees them do it? Whats to stop that person from trying to kill the observer? They can't be punished any further, right? In fact, there situation can only get better, because now they can remove the witness and potentially not be punished at all.