Where are you getting this info from?
If there's a price mistake on Amazon and a thousand dollar item is listed for $1, and I buy 10,000 of them, I can get arrested?
Law is a prediction of how the court will rule.
In your example, I am highly skeptical that the court would rule that theft.
Now suppose you had entered coupon code that you knew you were not entitled to use, like it said UPS Employees Discount. I'm not as skeptical anymore that a court says you knew you were doing something wrong.