Cramming is illegal but there are two reasons why enforcement is lax: First, as the article points out (and attempts, by inference, to get the carriers to convict themselves of), the billing carrier earns a cut, usually in the 20-30% range. Second, the individual amounts are usually quite small so it's not "worth" anyone's time to actually investigate unless that person is the subscriber who got dinged.

Simply shutting off third-party billing would kill off all kinds of pretty revenue streams that U.S. mobile and wireline carriers just love, such as purchasing apps from the Android and Windows Phone marketplaces without a credit card and "accidentally" (or intentionally, since they are somewhat popular) signing up for premium services via SMS.