[uulbiy]

IP spoofing can be prevented by ISPs[1] but I don't thing there is an incentive for them to do it. That could not stop the NSA anyway.

[1]: http://security.stackexchange.com/questions/1062/why-dont-is...

[SCHiM]

Also some (many?) ISPs have asymmetric routing, preventing an easy implementation of anti-spoofing mechanisms.