Maybe you are allowed to add those default routes, but IDF1/2 will think: whatever network I have to reach, throw the packet to default gateway 10.1.3.1, but where is that IP address?, does it belong to a network where I have a virtual interface?, the answer is: No, I only have a virtual interface on 10.1.1.0/24, I do not have a virtual interface in network 10.1.3.0/24, so I cannot throw the packet to next hop 10.1.3.1, so the packet is discarded.
Hope this helps you. Please give some extra kudos.
Pedro