Great question but you already know the answer.  Each state has different laws, different civil rights and different requirements for how laws are to be enforced.  What we are discussing is Denver's unique legal system. (not sarcasm)  While some ICE officers may be licensed police officers in the State of Colorado, it is certain that not all ICE officers are licensed police officers in all 50 states and in the various territories and the District of Columbia.  and there are statues that punish officers for screwing up. ICE would not want to assume liability for their agents screwing up in dealing with state and local laws.  So they would not let their agents enforce state law while on duty.  

Is that enough of an answer?