because you might be a terrorist haxxor out to destroy capitalist society. In terms of declaring the legality of the software on it, you'd probably only need to do that if they got picky. One thing to note: US Customs staff seem to have their sense of humour surgically removed - do not attempt to make jokes about anything, or they will single you out for attention.
You can be arrested for possession of terrorist materials (which is probably the intent of this), plus I'm sure the RIAA/MPAA and other interested industry groups are probably working on legislation to get themselves access to that information.
The US border checks are getting so intrusive that it will (if it hasn't already) damage the US tourist industry.
If you want to take electronic information into the states, and need a laptop over there (and are that paranoid), take a clean install laptop (i.e. no data on it) and post yourself an encrypted USB key with your data on it.
