It could just as easily come down to simple costs.

If it's less expensive to ask someone to hand over the data (in bulk) rather than burn CPU cycles cracking SSL (again: in bulk), then go for it.

Even if it's feasible to crack SSL for a few crucial messages, it's likely not so for the volumes of data the NSA are capturing.

If they can decrypt some SSL (maybe low bit) it is likely a very intense process that requires vast hardware resources, so even if they can do it, it is not likely it is being done for all traffic, but could be applied to some traffic.

Doesn't necessarily mean they don't have the ability to decrypt, it just means that they have a process in place so in case shit ever hit the fan (like it just did) they can come back and say "what's the big deal? we have a process in place"

Okay, tinfoil.