Why is the Japanese government considering adding kanji such as “cancer” to the jinmeiyō kanji?
I do not think that the government is trying to add these kanji to the set of jinmeiyō kanji.
I think that some people are confused by the unclear description in Wikipedia. At least I was confused at first. So probably it is useful to clarify it.
Article 50 of the Family Register Act (戸籍法) regulates that a name of a child in a family register must consist of common and easy letters, and the Ordinance for Enforcement of the Family Register Act (戸籍法施行規則) regulates that “common and easy letters” here means hiragana, katakana, jōyō kanji, and other kanji letters listed in the appendix of the Ordinance. “Jinmeiyō kanji” (人名用漢字) usually refers to kanji letters in this appendix. (The English Wikipedia refers to both jōyō kanji and the kanji letters in the appendix of the Ordinance as “jinmeiyō kanji,” but I think that this definition is less common.)
The quoted paragraph of the English Wikipedia is about the situation in 2004. The Japanese Wikipedia is much clearer on what happened in 2004. Partly motivated by the public demand of expansion of jinmeiyō kanji, the Legislative Council (法制審議会) of the Ministry of Justice published an initial proposal to add 578 kanji letters to the set of jinmeiyō kanji on June 11, 2004, and called for comments. This proposal was controversial because the proposed kanji letters included some letters which did not seem appropriate for names at all, such as 糞 (excrement), 屍 (corpse), 呪 (curse), 癌 (cancer), 姦 (rape), 淫 (obscene), 怨 (grudge), 痔 (hemorrhoid), 妾 (mistress). The reason for this is explained in the quoted portion:
This is because no measures were taken to determine the appropriateness of the kanji proposed, with the committee deciding that parents could make such decisions themselves.
In other words, the Council chose the kanji letters which were “common and easy” as the law regulated, irrespective of whether they were suitable for names. There was nothing wrong about this as long as the law is concerned, but it was different from how jinmeiyō kanji letters were chosen before.
According to the English Wikipedia, this made some people wonder “What is the point?” and made them speculate that there was a hidden reason for the proposal: the Council wanted to expand the jōyō kanji list, and the expansion of the set of jinmeiyō kanji was meant to lead to a de facto expansion of the jōyō kanji list. Note that the jōyō kanji list is maintained by the National Language Council (国語審議会) of the Ministry of Education, Culture, Sports, Science and Technology, which is a different organization from the one responsible for jinmeiyō kanji. I do not know if this story has any truth in it.
On the basis of the feedback from the public, the Legislative Council removed the nine letters which I listed above from the proposal (and added one letter) on July 23. They removed other 79 letters on August 13, and made this list as a final proposal on September 8. This final proposal came into effect on September 27, 2004.
Since then, the set of jinmeiyō kanji was updated a little in 2009 (and also in 2010 because of an update of the jōyō kanji list; remember that jinmeiyō kanji means kanji letters which can be used for names other than jōyō kanji). But I do not think that there is a current movement to add the letters such as “cancer” to the set of jinmeiyō kanji.