China did the same but for bigger scale in the past. The reason to do this is to store all traffic for future analysis. Now it is protected by https but there is no protocol or cryptography which couldn't be exploited in the future.
But as we all know, NSA does store all traffic for future analysis. A BGP leak from China or Russia, be that as it may, almost surely has nothing to do with storing traffic.
Nothing in Snowden's docs says that the NSA was collecting and storing all the traffic it could find for later analysis, which was GGP's unsupported claim.
That only protects against breaches of the long-term key, not against attacks on the cipher itself, AFAIK. So if in the future they manage to attack AES (or whatever cipher the connection was using), forward secrecy won't help.
If you use 1024 bit DH with a common group (old/misconfigured web and email servers do this) then it is suspected nation states can break the DH, get the shared symmetric key and decrypt all traffic.
For ECDHE over P-256, they would need to wait for a big quantum computer (which will break all recorded traffic that used a non-quantum resistant key exchange, which is all current traffic).
But as we all know, NSA does store all traffic for future analysis. A BGP leak from China or Russia, be that as it may, almost surely has nothing to do with storing traffic.