Thyroid abnormalities aren't necessarily cancerous. Only a very small percentage will be malignant, and as far as I know, the rest of the nodules do not necessarily cause any symptoms.

Although that high a percentage of thyroid nodules sounds shockingly high, it's not clear if that is actually abnormal for Japan. There are other thyroid surveys that people reference where the number of thyroid nodules is very small, but the definition of what an abnormality is varies significantly between surveys so it's an apples-to-oranges comparison. There are surveys planned or already underway in supposedly uncontaminated parts of Japan to obtain a baseline that the Fukushima results can be compared to.

I say "supposedly" uncontaminated because Tokyo has arranged for mildly radioactive disaster debris to be transported throughout Japan to be incinerated. These facilities are designed to burn normal municipal waste, not hazardous materials, so it's not clear how much radiation will escape to contaminate other parts of Japan. This has been going on for some time already.

A so-called conspiracy theory is that "they" want to raise the level of radiation throughout Japan so that more people outside of Fukushima develop health problems similar to those in Fukushima, thus making it seem like the nuclear disaster was less harmful than it was. Sounds crazy, but I suppose no more crazy than a government that would deliberately contaminate more areas and more citizens with radioactive isotopes that will be around for thousands of years.