The Kaon disintegrates into a Pion, a neutrino and an antineutrino. This process is rare because you need two virtual Z or W particles in the intermediate steps of the decay.
It's more that they've set up a marker that says "We're getting good at measuring the K+ decay, which is super-rare, and we are trying to establish a good frequency for it. Anything new physics proposed has to be within a certain error range of this result." It's more of a way to quickly rule out "new physics" models that do not pass the test than anything else.
In some sense, many of these measurements are ways to provide more points in some kind of line-fitting we have going on. "Sure, your new idea waves away dark matter buuuuuut there's this little matter of its predicted K+ decays. They would be way off. Either come up with a way to explain that or try again."
The Kaon disintegrates into a Pion, a neutrino and an antineutrino. This process is rare because you need two virtual Z or W particles in the intermediate steps of the decay.