In most cases, one could purchase the second ticket on another carrier, which DL obviously would not prefer that customers do. So they're willing to let those with price elastic demand purchase the trip as two separate tickets to save money. Others with less price elastic demand and business travelers as well as those who aren't aware of this "trick" or don't notice the fare anomalies will just purchase a single more expensive ticket. It's similar to offering a coupon or rebate offer which will only be taken by customers who are more price conscious.