As of Tuesday, 26 July 2005, the piracy check will be mandatory for all customers worldwide who want to download add-ons for Windows XP.

Read more about how M$ is out to get you:
http://news.zdnet.com/2100-3513_22-5804045.html?tag=nl.e589

Probably not, but those who can't be bothered or are to simple, or simply don't know to patch their genuine systems would be awfully upset if they got a worm or a virus as a result of systems that are unpatched beacuase they are pirated.

Of course, it could also be MS doing it's bit to try to halt the ever increasing amount of malware out there - but methinks my first idea is more plausable.

Well, that is entirely true, but given how fickle the human population is, I would probably assume that MS would not wish to give such people any further excuse for them to blame them for it. Many such people will often put blame on anyone they can, as long as it isn't themselves.

Considering the fact that you can be sued if someone breaks into your house illegally with intent to steal things from you, then gets seriously hurt due to a hazard inside your house...

It is probably more of a liability issue than anything.