There is an excuse for not "securing" the network: disallowing spoofing is not desirable in the first place. There are legitimate uses to spoofing. What is not desirable is malware infecting user computers and using spoofing for DDoS attacks, but the ISP cannot know whether the packets were sent by malware or by a user. The proper way to solve the issue is to make secure systems that will not get infected easily by malware.