The WaPo report nicely sums up the issue: "What’s happening in restaurants is just a trickle-down effect of what has happened to dating." Basically, as more people opt for inexpensive dates involving a few drinks instead of settling in for a pricey multi-course dinner, restaurants are being forced to no longer rely on the typical business model of parties eating, drinking, and leaving so the next group of people can do the same. It's worth noting, however, that the National Restaurant Association doesn't have any data on exactly how dating has impacted the industry.
Some restaurant owners, like Ashok Bajaj in Washington D.C., are replacing booth seating with more tables designed for two people, which allows the restaurant to seat more more couples, per the report. Obviously, not all restaurants are complaining about the high volume of such dates, because after all, some business is better than no business.