Also, is his concern about earthquakes really warranted? I'd have thought that a DC would have been able to withstand more or less anything thrown its way.
I think this is an NFPA/NEC[1] standard.
You could look at it another way: circuits are breakered such that you can temporarily peak 25% above their normal capacity without tripping the breaker. I don't know why the standard isn't more conservative, except to guess that it's because typical household/office uses are likely to have low continuous and high intermittent loads (e.g. vacuum cleaner), making it a good trade-off.
[1] USA (and Canada?) only. Anyone know how it is in other countries?
But kika's downthread comment is most on the mark - of course data centers would rather advertise the less relevant "circuit rating" as the number is higher. The load calculations are the same either way.
(btw speaking of the NEC, http://bulk.resource.org/codes.gov/ is a fantastic resource)
I'm not sure why this came about in the first place, though. AFAIK in Europe breakers are rated at 100%.
The breakers in your home breaker box operate the exact same way. A 20A breaker will trip once you exceed a certain threshold, which I believe is 16 or 17. They're rated to go to 20 by the factory, but to prevent problems with overloading and causing fires/explosions/etc, they purposefully break at a lower threshold.
So this isn't just a colo price scam thing, this is the way the world works.
2. A 20 A circuit breaker is allowed to trip at 20.000001 A. Derating the continuous load provides the safety margin for reliable operation, even using inexpensive circuit breakers.