In the case of ACM, we don't even have those costs. The conferences and reviewing is entirely community managed. The answer is: it's taxpayer and company funde

In the case of ACM, we don't even have those costs. The conferences and reviewing is entirely community managed. The answer is: it's taxpayer and company funded, in an indirect way, by researchers contributing their time to do it. There are no paid editorial staff. These days, there's increasingly less paper involved, which eliminates the paper printing part of it as well. (those folks were paid, full time printers - but acm typically outsourced it to a printing firm anyway). Very little remaining justification for copyright assignment to publishers.