Reason is, the atomizer gets hot fast. And when the atomizer is hot there is a danger of burning it out. Typically when you are on a battery you are more aware of how much you are using it and can limit the burn time to prevent atomizer failure. I found when on USB I tend to fry atomizers on my pen style more often by over heating it.
I had the same concern; when I put together my first 901 passthrough, not only did the atomizer get incredibly hot in a short amount of time (and ate juice like mad), but it also made my laptop screech that I was drawing too much power through my USB port.
I'm an amateur when it comes to electronics, but I did a whole slew of research and dug through this forum. After all that, I found out that putting a 1N4004 rectifier diode in series with the power coming from the USB to the battery (anode side attached to the switch, cathode side attached to the lead going to the N Coaxial power connection) dropped the voltage by about 0.8v.
The result is that the atomizer is only getting the amount of power it would from a freshly charged battery, and it's not drawing as much electricity from the computer. The laptop stopped giving me the warning, and the atomizer is only as working as hard as it would be attached to a battery.
My only remaining step is to seal the N Coaxial DC jack with some hot glue, because the jack has an open spot and draws way too easily for my liking. I plan on putting a ballpoint pen cartridge over the connection point to keep the glue from getting on it, and then filling around it. Should solve the draw problem . . .
Anyway, I'm off the point. If you're concerned about frying your atomizer, just add a 1N4004 diode. You can pick up 2 from any Radio Shack for a mere 99 cents.
Nash