They're holding the seats for international connections. For example, on June 14th there are at least 6 seats available if you search FRA-IAD-IAH:
Code:
1UA 988 F3 A3 J9 C6*FRAIAD 1220P 305P 777 L 0 DCA /E
D6 Z0 P0 Y9 B9 M9 E9 U9 H9 Q9
2UA 249 F3 C6 A3 D6* IAH 8 520P 752P 739 D 0 DCA /E
Z0 P0 Y9 B9 M9 E9 U9 H9 Q9 V9