Read the article. Their rationale is fairly clear: They want slower bikes and reduced risk. "Insurance" is also mentioned a couple of times in the article.
Reasons mentioned include the fact that the course times had plummeted after it was fully paved and bikes like CBR1000RRs and ZX10Rs began running it, not just machines like the Duc 1200 Multistrada.
To be fair, it would hardly be the first racing series in the US to see measures put into place to limit speeds and reduce risk. Look at NASCAR restrictor plate tracks, for example. Or IndyCar where reduced turbo boost levels were mandated after crashes during qualifying. They made other rule changes to increase downforce and reduce power levels.
This is the United States, lawsuit capital of the planet, not the Isle Of Man. I'm surprised they have such a race like PP there at all...