They explained that pretty clearly. The US insists that you are entering their country if your route of travel passes through US territory. So sensible routes to lots of other countries that don't themselves have obnoxious rules are off limits if you don't want the US nosing thorough everything.
If your social network accounts are public, the NSA can already see them, it doesn't matter if you explicitly give them your username.
If your accounts are not public, giving them your username won't really do anything.
I'm still not sure I see the problem? "Oh no, I'm traveling through the US and now they know that I have a social network account!" ?? They already knew that if you were actually a person of interest.
But the TSA and whatever other relevant departments don't have access to the NSA databases (or don't publicly show it, same thing from my POV).
I perceive that I might be denied entry to the US because someone shares a photo of me doing something the US doesn't like .. Then my trip to Canada gets cancelled, and I get to say "yes" when asked by every other country "have you ever been denied entry".
Meanwhile I must also provide biometrics.
And it is a federal form. The theoretical risk is high (although actual risk is low if you are a normal person).
However it sucks that the US is the easiest/cheapest way to transit to many countries.
However I can't transit through say LA without "entering" the US.
So I can't visit other countries easily (especially Canada, or central America).