Law enforcement uses dragnet warrantless surveillance to gather data (such as who is calling whom, who is in physical proximity with whom). That data is loaded into Palantir's software.
Law enforcement uses that data to acquire admissible evidence of the perpetrators' crimes. This is the crucial step, because although the original evidence couldn't be admissible in court due to its collection without a warrant, it can be used to acquire more evidence. If the warrantless surveillance includes information such as the time and place of where you plan to commit a crime, law enforcement can stop you for a random check on the way there, giving them actual evidence for prosecution. Now that evidence can be used in court, even though the only way law enforcement were able to acquire it was through unconstitutional practices.
The thing about Palantir is that it needs big data to work. Police departments don't just have random chunks of data lying around containing large numbers of connections between large numbers of people - they might have something like that for something like suspected criminal gangs, but that wouldn't be enough data to make Palantir's software worthwhile. So acquiring the data necessary to use Palantir's software requires mass data collection. Some of their mass data collection techniques may not require a warrant, such as using license plate captures, but some does, such as who you're calling.
So does Palantir perform warrantless surveillance themselves? To my knowledge, no. But when they sell their software to law enforcement agencies, they must know that the only way their customer could get bang for their buck is through mass surveillance. In my opinion, the person willingly and knowingly selling tools for oppressive purposes holds a lot of blame for the actual oppression that occurs as a result.