In the U.S., the act of thermally imaging your home IN AND OF ITSELF constitutes a search and is thus illegal without a warrant. That's been ruled on by the Supreme Court, so that is the law of the land. In other words, they can't use thermal imaging to obtain a warrant; they must have a warrant first, for some other reason (leaves in your trash, see a bong through your window, something). And obviously, once they have a warrant, then it's cheaper and more effective (and probably more fun for the cops) to send a couple squad cars over and tear your house apart, rather than use an expensive helicopter and thermal imaging equipment.