Isn't the issue that the designer can choose to work for less than minimal wage or not work. That's a very limited sense of consensual labour.

The exploitation of the global market is available to the commissioner (stackoverflow) in a way that is not available to the designer.

Whilst SO are enjoying a position of wealth wrought in part from the [owners] geographical location they are, here, using the lack of mobility of designers to get a relatively low cost design created. Moreover they are exploiting those who, on the whole, don't have the benefit of their wealth in order to maintain a low cost and at the same time creating a vast inefficiency in the labour performed to produce the design.

So the immorality IMO comes in via exploitation of the low paid, deflation of market price below local [labour] costs (as they are benefiting disproportionately from those high local costs), and purposefully introducing inefficiencies in to production.

You may not find those condition immoral - after all population control by virtue of below-subsistence wage levels is a natural part of a free market; some people seem to find that satisfactory.

If that's what you call immoral, do you only buy goods and services produced by workers making American/Western European-level wages? In the unlikely event that you do, isn't it morally suspect to deny workers in developing countries jobs just to maintain wages?