The ban will not affect the new Galaxy Tab 2 10.1, which is in the shops now, so Samsung will not suffer that much.
Koh wrote that although Samsung has a right to compete, it did not have a right to compete unfairly by flooding the market with infringing products.
She said that Samsung will suffer lost sales from the issuance of an injunction, but Apple will suffer if it had to directly compete with Samsung's infringing products.
It is a preliminary injunction and Samsung will certainly appeal.
It is not clear what has changed Koh's mind. She denied a previous Apple request for an injunction against the Galaxy Tab 10.1 last December.
That decision was turned around by the US Court of Appeals which said that Koh was incorrect in thinking that an Apple patent related to the iPad may be invalid and said that Cupertino was well within its rights to ask for a ban on sales of the device.
Apple has had to post a $2.6 million bond to protect Samsung if it is later determined that the injunction should not have been granted.