Explanation/Reference:
L1 routers use L1/L2 routers as the next hop for the default route 0.0.0.0/0. The L1/L2 routers do not advertise a default route-instead, the L1/L2 routers set the attached bit, which is advertised to the L1 router and causes the L1 router to install a default route with the L1/L2 router as the next hop.
If an L1/L2 router does not have an L2 adjacency to a different area than its own, the L1/L2 router will not set the attach bit, resulting in the L1 routers not installing a default route.
References: Network Scaling With Bgp Labeled Unicast, Design and Configuration Guide, page 8