That is the first point of contention. The protestors might have eventually stopped federal agents from enforcing immigration laws, but in the immediate and short term they were only preventing them from legally entering their workplace.
The “law” usually looks at the immediate cause of your actions, not the long-term and abstract result of your actions. When you give a homeless person money, for example, you might be enabling a drug purchase, but you would never be arrested for aiding a felon.
In the US, federal and and non-federal are separated.
Immigration is in federal law jurisdiction. States/local agents are not required to enforce it. In fact, the federal government prohibits states/local governments from passing their own immigration laws.
Similarly, the federal government cannot coerce the local authority into aiding them with the execution of federal law.
The division of power in the US can make for interesting cases. Immigration is a VERY interesting one, because of how the powers were divided.
This, however, gets to the main point.
If you protest illegally(generally trespass in your protest), you will be removed by local law enforcement. This is not an endorsement of the group being protested by the local police.
It upsets me when people fail to make the distinction between “following the rules” and “tacit endorsement”.
This is the same silliness that makes people hate the ACLU.
