I'm not quite sure where you get your information from, but I seriously doubt any providers are specifically rewriting spoofed udp packets into legitimate addresses. Routers simply do not perform tasks like that.

Providers are not dropping/rewriting just spoofed udp. They're either dropping all spoofed traffic, or letting all traffic through. Why would they apply this rewriting only to udp if they were indeed doing it? The simple answer is that they are not doing it.