Maybe it's a little bit hard to visualize, so I added a image:
In the 10.1.2.0/24 there are requests with 10.0.1.0/27 as source IP and these packets are routed to the default router, I have no chance to add static routes or somethin to choose the right way back.
If the packets are source-natted with the IP 172.17.1.1 (Interface of the ASA), the way back is correct.
HTH, Rene