Surely it can't have been designed like that? Can it?
By default, iTunes is set to update iPod automatically. When you connect your iPod to your computer, iTunes opens, copies selected songs in your library and playlists to your iPod, and erases songs from your iPod that aren't in the iTunes library. Depending on the type of iPod you have, you may also be able to create and manage playlists on your iPod or synchronize contact and calendar information with your iPod.
You can create a standard playlist or a Smart Playlist. Standard playlists are created manually and are not automatically updated as your library changes (exception: songs you delete from your library are deleted from the playlists that include those songs). Smart Playlists are created based on rules you set up, and are automatically updated as your library changes.
To update iPod with only certain songs:
1 In iTunes, check the boxes next to songs you want to transfer (by default, all songs are checked).
2 Select iPod in the iTunes Source list and click the Options button.
3 Select “Only update checked songs.”
To set iPod to update only selected playlists:
1 In iTunes, select iPod in the Source list and click the Options button.
2 Select “Automatically update selected playlists only.”
Transferring Songs and Playlists to iPod Manually
You can set iPod to transfer songs manually, so you can transfer individual songs and playlists. This is especially useful if you want to use iPod with more than one computer.
When iPod is set to transfer songs manually, iTunes won’t update it automatically when you connect it to your computer.
With your iPod connected and selected, select edit > preferences, then under the ipod tab deselct automatically update and select manually manage songs.