The link doesn't really say why spec work is unethical. It seems like they are frustrated because designers can do a bunch of work and then not get paid if they don't win the contest. That doesn't seem unethical, that's just how the system is set up. If it isn't a good deal for you as a designer because you can get better jobs easily, then don't participate in these contests.

Personally (I'm a programmer) I would never do work for someone else with payment contingent on whether they liked it on not. But I don't think it's evil or unethical to do so, it's just not a good deal for me, so I don't do it.