As an EE, I find that hard to swallow.

As long as, within reason, the proper voltage is being supplied to the USB port, then it is up to the charging circuit to determine how much current it will draw. If you hook a 5V supply to a 25 ohm load, you'll consume 1 Watt from any supply can source 200 mA or more.

There is some complexity with internal resistance of the supply, but that is a secondary effect which, if anything, a "smart" charger could be aware of, dropping its current demands when the supply voltage starts to droop, hence automatically accommodating everything from the poorly-regulated CLA from the guy with the port-a-table on the corner, through my 350 mA Plantronics charger, to a Nokia OEM charger can can source 1200 mA.

That the N900 allows "shorted data" USB supplies to bypass the handshake contraindicates the hypothesis that this is somehow supposed to "protect the charger."

I find it reasonable that Nokia is worried about the case where

I hooked my N900 to my laptop and it fried my $3000 laptop.

but find it unreasonable that they are worried enough to "lock out" virtually all non-active-USB-hub devices (including problems with their own DC and AC adapters) for the case where

I hooked my N900 to my no-name CLA and it fried the CLA.

which generally would be prevented by any well-designed "smart charger" circuit (which would be looking at the charging current and voltage both),
or even

I hooked my N900 to my no name CLA and it cooked my N900.

"Well, what did you expect?" would be the polite form of the comments someone would likely get in the latter two cases. In neither case, if the charger had the data pins shorted, would the apparent N900 behavior, have saved the charger or the N900.