In: Economics
To answer this question let's first understand what does rule utilitarian perspective and sweatshop means.
The utilitarian perspective appproaches any action in terms of its consequences or final outcomes, that is to say the bet benefits and costs to all stakeholders on an individual level. It focuses to achieve the greatest good for the greatest number while creating the least harm possible or prevent from greatest amount of sufferings. It assumes that every entity's interests should be considered equally when making a decision, this also includes other living species which is capable of any possible suffering.
And now let's see what exactly does sweatshops mean. A sweatshop is reffered to a working place which is in a very poor condition with rigards to basic human amenities. It's where people are made to work for hours with proper working environment at a very low wage, in other words complete exploitation of workers.
As you can already see, that rule utilitarian perspective and sweatshops means two different things and can't coexist at the same time.
Now coming to question, whether it's ethical or not for the international fashion brands to manufacture its products in a sweatshops at a very low cost from the perspective of utilitarian. So you might already have understood by now, that it is highly unethical from the rule utilitarian perspective for international brands to manufacture their products in a sweatshop at very low cost since they are exploiting the labors at sweatshops for their personal gain which is highly unlikely from utilitarian point of view. In this case all the suffering goes to the labors working at sweatshop and all the benefits and economic gains goes to the owners of international brand, which is not what utilitarian perspective suggests.
So, it is highly unethical for international brands to manufacture their products in a sweatshop at very low cost from the viewpoint of utilitarian perspective. Since the gains and suffering from the economic activity of international brands are not being shared equally among all the participants. Labors at sweatshops gets all the suffering and owners of international fashion brands gets all the benefit.