As I read on Section 4.3.8 of this page https://www.manrs.org/isps/guide/antispoofing/,
The following code:
ip nat inside source list INSIDE pool OUTSIDE overload
will translate packets with a source address in access-list INSIDE and change the source address to an address in the pool OUTSIDE. However, packets that have a spoofed source address that is not included in the INSIDE access list will be forwarded without any translation, resulting in spoofed packets on the Internet
If the claim made on this page is true, why doesn't NAT drop packets that are not present in the INSIDE list? If this can be done then we can reduce spoofed IP addresses on the internet.