In order for the government to obtain a warrant, they would need some sort of probable cause. Most of the time, that probable cause is justified by finding whatever bad thing they were looking for, and the bad person is punished accordingly. The unfortunate thing here is that once they have the ability to hack in to one computer, what is stopping them from doing so to all computers and who is going to keep the government in line with this ability to hack. Could I see this happening in the future? Of course as we integrate computers in to our lives more and more everyday. If it does happen, there will need to be some sort of checks and balances system set up to ensure the government does not abuse their ability to hack and is really only using that ability for the overall good.
Yes, in theory the U.S. government should be able to obtain warrants to hack into computers throughout the world if there is a legitimate security concern. The real question is: from who would they receive the warrant? The U.S. courts don't have a right to issue a warrant in another country. Would the foreign government have to grant the warrant?
The United States has a problem. That problem is called terrorism. There are lots of people around the world that think that if they kill enough innocent people, Americans will give in and live the way they want them to. If America can obtain warrants to search computers and defend themselves, that is great for them.
The US jurisdiction should be limited to the US borders. Hacking is an international problem that crosses borders and therefore should be addressed by an International approach. If the US were to come up with its own rules for issuing warrants for international hackers, those warrants would be subject to the cooperation of another nation state and would only be enforceable if that country agreed. If the US and other countries set up an international agreement regarding warrants then the participants to that agreement would have an enforcement mechanism that the unilateral issuance of warrants would lack.