I have 5 Wiimotes and two see heavy use each day. So my brother and I both bought 8 rechargables.
The problem with rechargable batteries are if you get bad ones they have tendency of leaking, starting fires, or exploding. (Just ask Sony how bad, bad recharables can be. ;P ) Nintendo is just covering themselves from liablity if someone bought bad recharables and something happened they then couldn't go an sue Nintendo. Now a days a simple cup of hot coffee comes with a warning. (This also goes to help prove that the average intellengence of society as a whole is dropping rapidly.)