A union does not necessarily play the role of migitation of negative externalities from a company's profit seeking activies. It still wants to maximum profits b

A union does not necessarily play the role of migitation of negative externalities from a company's profit seeking activies. It still wants to maximum profits but with workers taking a fair share of such profits and under good work conditions. For example, unions that represent workers in dirty industries are very often resistent to regulation of their employer's activities despite the harm it can cause to others. Coal mining, steel processing, etc... They don't even necessarily lead to good value for money for consumers - for example they could use forms of worker accreditation to reduce the supply of workers to raise wages for existing workers and the increase the prices their employers can charge.