It takes similar resources to prosecute the thieves too, and that employee who can steal the good stuff can probably steal a whole truckload you know.
For the most part Wal-Mart's security system is sufficient to stop almost all petty theft anyway.
"333 kids stealing a $3 pair of socks."
Think about this scenario for gang recruitment, where the gang wants to engage in organized shop lifting.
Send in new recruits under 18, under $25. If the kid is caught... no arrest, no record... if the kid isn't caught, he can now move up to bigger shoplifting tasks in the store.