What possible motivation would employers have to shelter a lawbreaking employee, at no benefit to themselves? Why would the threat of violence be remotely necessary?

What makes you think that? As far as I know, there is no easy way for a third party to determine whether any given company cooperates with garnishment orders, so how can this incentive exist?

> and because they have accepted the proposition that paying a reasonable amount of tax is a civic duty.

I imagine that most citizens think some level of taxation is a civic duty. But I also suspect that a large portion of citizens think their own level of taxation is too high, and would give less to the government if the government changed its rule to "give us whatever you think is appropriate to fulfill your civic duty."