They may not want to leave a paper trail.
“To obtain a [legal] wiretap order, law enforcement must demonstrate probable cause to a judge” [1]. Given ICE has been arresting Americans, they probably aren’t bothering with cause. Legal intercepts also require “minimization procedures to limit the interception of conversations unrelated to the investigation,” which ICE may not want to do. And perhaps most importantly, “violations of 18 U.S.C. § 2511 are considered federal felonies,” with those convicted facing criminal penalties and being subject to civil liability.
I hate to make this analogy, but it’s akin to the Gestapo’s NBH obsession. They knew they were acting illegally, and didn’t want to leave the evidence that would convict them.
[1] https://legalclarity.org/what-is-wiretapping-and-when-is-it-...