I don't understand your point here. Are you saying compliance isn't enforced?

As someone who got an ACS survey not long ago and had no interest in completing it, it certainly appears to be.

There's not many cases of enforcement. Non-response is taken about as seriously as the Robinson–Patman act. I think the Census Bureau is very reliant on people thinking there will be enforcement, however, which is why the materials they send all have a threatening aura. I don't know about the ACS, but for the decennial census I often felt like my job as an enumerator was just to bother people until they'd answer. The case would keep being recycled until we got at least (IIRC) a head count.

They can certainly enforce that you answer the survey. But it's very difficult to enforce a requirement that people answer questions accurately, particularly when they perceive that doing so will expose them to danger.