It's a question of incentives. From what I've heard: ICE agents are incentivized to the tune of $5000 for every immigrant deported. So they go after the low-hanging fruits: the immigrants coming in for their periodic immigration court hearings, the Home Depot parking lots, etc.
This is why you hear about old grannies being arrested and deported and random immigrant workers with no criminal history being nabbed.
Basically, ICE is a group of bounty hunters and they have no qualms about breaking the law if it leads to a nice payday.