This is essentially the same as if you got sued in civil court for doing something (say assault). Now imagine that your bank account is pretty much empty and all you really have is your car. The court can take your car to satisfy the judgment against you. Of course, this particular case is different, owing to the diversity of citizenship of the parties. I imagine that Umbro tried to assert that the court had jurisdiction over the defendant's assets in Canada, and the garnishing of the domain names was an attempt to find something of value.

Too bad I read this only after coming from my Trademarks class. ;-)