It depends. There are a lot of different cases here

I'm not a lawyer, but damn, some days I think I missed my calling. Without doing much in the way of due diligence, my understanding is:

Technoviking wouldn't have a case (if the festival had been in the US) since anything you do that can be filmed in a public space, or in some instances, from a public space (where you don't have a reasonable expectation of privacy) does not require your consent, and that was filmed at a public festival. There is some complication around Model Releases depending on how the depiction of Technoviking is going to be used – for example, the difference between posting the video on YouTube and licensing the same video to a bottled water company for advertising – but (again in the US) obtaining the Model Release from Technoviking would be the bottled water company's responsibility, not yours.

The creators of Nyan Cat and Keyboard Cat both retain control over the use of the intellectual property that they created, and can sue the crap out of unauthorized users.

Star Wars Kid – the video is not really actionable in the US by itself, but the distributors can (and were) sued for various harm provisions that would fall under the general category of slander/libel and recovery of money spent on therapy and medical care (prescriptions, etc.) that would not otherwise have been spent.

I don't know (and I don't know that anyone knows for sure) what the legal standing would be for using the unlicensed likeness of Grumpy Cat would be – particularly if an unrelated artist drew the likeness from a blank sheet of paper, rather than modifying a photo (which would be copyrighted material by Tard(ar Sauce)'s owners. The question was raised in 1954 as to whether or not animals (Grumpy Cat, Lassie, etc.) have likeness rights the way people do (Kat Dufoe, Liv Tyler , etc.), but it hasn't ever been challenged to resolution in court.

Reshared post from +Jason ON

Righteous indignation or spoil sports? I'll grant, not all of there's can be judged equally, for example the down syndrome kid, but once something is released to the internet aethyr, it's free from your control.

As the Technoviking filmmaker put it, these things only become more popular when people make a fuss about them.