Say... You want to buy one of these notebooks, and don't want to pay for XP. Ticking the Unbuntu tick box saves 40-50$ (just double-checked vs. the dell site). You then install XP yourself. I'm sure drivers are available from the Dell website (or pherpas even come as a kit). Dell doesn't have a "no-OS" (barebones) option.
I'm an avid Linux user (writting this post on a Hardy Heron Ubuntu, actually). However, I'm also realistic.
On the other hand, there's nothing wrong with copying bits. Laying ownership to a sequence of bits is just plain silly.
May all your PUSHes be POPped.