There are 2 issues here and it's important to keep them separate.
1: If you are in violation of the license, must you put your code under the GPL? The answer is definitely no. If you are in violation of the GPL you may license your code under the GPL to become compliant (though in V2 and below you also have to be forgiven by the copyright holder(s)). This is an option that you have in addition to the options you have for any other copyright infringement (i.e. going to court). You never have to relicense your code. It's just a way to avoid court. There are definitely people who misunderstand this point, but I don't think you'll find anybody in an official capacity at the FSF who doesn't agree with the above.
2: If you write code that complies with an API in an GPL project, do you need a license? Like I said near the bottom of my other post, that's the grey area. Some people will say yes and some will say no. The idea is that complying with the API necessarily means that you are creating a derived work and are therefore infringing copyright if you don't have a license.
Like you, I initially found this idea to be very strange. However, the Oracle vs. Google court case lends considerable credence to it. In that case Google was found to have infringed on Oracle's copyright by implementing a published API. The judge ruled that it was fair use, though. I suspect (IANAL, remember) that if this was ever ruled upon you would probably get a similar judgement. The situation is fairly different, though, so you never know.
Now, the reason you don't find people challenging this stuff is (I think) mainly because the copyright holders are clear how they stand on the issue. I don't know anything about it, but I'll take your word that this applies to WordPress plugins/themes. You can just go against their wishes, but it's kind of a jerk move. Will anybody sue you if you do it? Will you win a court case? Maybe, maybe not, but you will certainly upset a lot of people so why do it? With the Linux kernel and binary blobs, it's all good as long as you stick with certain APIs. People do it knowing that nobody is going to sue them. NVidia steps over the boundaries, gets yelled at by Linus, but still nobody sues them.
I mean, you can also look at the really weird interpretation of the AGPL that projects like Mongo DB have. Their attitude is way more liberal than the license seems to state (to the point where I frequently wonder if they have actually read the license). But who is going to complain? It's their code -- they can do what they like. They don't have to sue anybody if they don't want to.