My guess is it is a response to all the batteries blowing up lately and the fact that Lithium Ion batteries have a habit of wearing out when maintaining a full charge all the time.

I've had PDA's, Laptops etc. that although they were "portable" they were plugged most of the time. The batteries can wear out being full all the time. IBM even had this listed on most of their laptops for a long time but didn't put it in such bold print.

Although there are circuits in most batteries to prevent over-charging they aren't fool-proof. Some like my Dell seem to charge to full but if you leave it plugged in the battery goes dead - so the circuit stops charging all together once full and doesn't start charging again until the device is unplugged and plugged back in.