The accused is accused of violating federal law, so it's normal that a federal agency would make the arrest. FBI seems to make more sense than DEA or ATF, no?
ICE being shady is by many people accepted, the DEA, ATF even. But the FBI has built itself a pretty strong reputation of integrity and professionalism, and resistance to political pressure and corruption. In some ways I at least viewed it as a firewall in law enforcement against this sort of stuff.
Now who watches the watchers?
FBI is prestigious because they get the most qualified tyrants, who are smart enough to lie and deceive in ways that are airtight enough that those at ICE take the heat. The surprising thing here isn't the fact that they did it, but that they didn't do the normal way of digging or manufacturing something else to pin on the judge.
The fact you can just fire the whole federal government (yes, i understand the probation thing) and there's *nothing* that blocks it is just completely bonkers to me. All you really needed was a bad actor that had no respect for the norms, because there's no real consequence to breaking them.
You can't just arrest someone for nothing. You need probable cause. The question is whether a judge going about their day, doing nothing illegal, is probable cause. It's very likely not.