>> If some guy's due to testify in a murder case, I don't want him skipping court because some quota-making jackass at ICE wants to arrest him because of a visa issue
From the criminal complaint in this case:
"I also am aware that pursuant to its policies, which had been made known to courthouse officials, the Milwaukee ICE ERO Task Force was focusing its resources on apprehending charged defendants making appearances in criminal cases – and not arresting victims, witnesses, or individuals appearing for matters in family or civil court."
So it sounds like they take this into account. As for why they make arrests at the courthouse:
"The reasons for this include not only the fact that law enforcement knows the location at which the wanted individual should be located but also the fact that the wanted individual would have entered through a security checkpoint and thus unarmed, minimizing the risk of injury to law enforcement, the public, and the wanted individual."
Makes sense. Seems like they have weighed the risks and advantages of this and come up with a reasonable approach.