It depends on exactly what services Schneier is talking about here, but an awful lot of the infrastructure of the internet is hosted in countries that the US armed forces have easy physical access to.

Even if a cyber attack were the "plan a" for quickly and untraceably taking those systems out, the US has an easy enough "plan b" that testing "plan a" isn't going to be a major concern. Add that to the fact that the US has a lot more to lose if it gets caught attacking internet infrastructure than China and friends do (even just tests like this) and I would be surprised (honestly not shocked, but definitely surprised) if the USA is behind these shenanigans.

Actually, if the US wanted to test something like this on a service in a friendly country, I would expect the NSA to approach the infrastructure company and say something like, "We're concerned that $enemy_of_free_speech may be planning an attack on your service, and we would like to wargame that scenario with you. What time(s) would an outage have a minimal impact to your bottom line?"