I assume you didn’t actually research this confident claim. Because it’s simply wrong.
If you had, you might have read about the case of Wadih EL-Hage, a US citizen living in Kenya in the late 1990s. Believing that he was involved in terrorist activities, US authorities wiretapped his Kenyan telephone for a year. This was done without any warrant. They also broke into his house and searched it, again with no warrant.
Evidence gained from that wiretap and search was used to convict El-Hage in criminal court in the United States, of conspiracy to murder United States nationals; conspiracy to murder internationally protected persons, United States officers, and employees engaging in official duties; and conspiracy to destroy buildings and property of the United States, all related to several embassy bombings.
On appeal, El-Hage made the same argument that you just did: the Constitution forbids the introduction of warrantless wiretap evidence and warrantless search evidence against him, even if the evidence was gathered while he and the phone in question were in Kenya.
The court disagreed:
(“Warrantless electronic surveillance has been used by the Executive to collect intelligence information since at least the mid-1800s … Warrantless physical searches have been used for a much longer period of time.”). Congress has legislated with respect to domestic incidents of foreign intelligence collection, see FISA, 50 U.S.C. §§ 1801 et seq. (1978), but has not addressed the issue of foreign intelligence collection which occurs abroad.
The court went on to extensively analyze the factors associated with warrantless foreign wiretap evidence, and concluded it was properly admitted against El-Hage in his criminal trial. The Second Circuit unanimously affirmed his conviction. In Re Terrorist Bombings of US Embassies in East Africa, 549 F. 3d 146 (2nd Cir 2008), cert denied, 130 S.Ct. 1050 (2010)]
So what the hell were you talking about?