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In these following examples, a few words are given furigana:  
[シドニー立てこもり、治安部隊が突入
人質？担架で救出](http://www.asahi.com/articles/ASGDJ02M7GDHUHBI044.html)  
[年賀状家族や友達へ投函](http://www.yomiuri.co.jp/local/saga/news/20141215-OYTNT50475.html)  
[金剛峯寺にゴボウ奉納
黒河道世界遺産目指し](http://www.yomiuri.co.jp/local/wakayama/news/20141215-OYTNT50291.html)

So, it looks like that furigana is provided when a 人名用{じんめいよう} kanji needs to
be read. That makes sense. But, here are exceptions:
[未](http://jisho.org/kanji/details/%E6%9C%AA)、[雑](http://jisho.org/kanji/details/%E9%9B%91)[事](http://jisho.org/kanji/details/%E4%BA%8B).
While giving furigana for straight-up 常用 kanji happens rarely, it does happen
and I can't detect why.

So, what is up when an adult newspaper gives furigana for 常用{じょうよう} kanji that
are not using [当て字]{あてじ} readings? What metadata is revealed when a (for a
native speaker) seemingly simple to read word is given furigana? Asahi and
Yomiuri are written for Japanese native adults. Why then given them an assist
with reading?
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furigana
What does it mean when newspapers give furigana for 常用{じょうよう} kanji?
Newspapers give furigana for two reasons:

  1. Because the word contains non-常用漢字, or _non-常用 readings_ of kanji. (This is a PC matter for public media. Novels never do it.)
  2. Because the kanji is rarely used, or the word should be read in local, irregular, nonce or other unexpected way to readers, or potentially ambiguous in pronunciation. (This is the traditional use of furigana.)

As I have no subscription accounts for those papers, all furiganas I see
through your links are 閃光{せんこう}, 幇助{ほうじょ}, 投函{とうかん}, 未{ひつじ}, 悠乃{はるの}, 雑事{ぞうじ}
and 背負子{しょいこ}. They are categorized into:

  * 閃光, 幇助, 投函 → #1
  * 未, 悠乃, 雑事, 背負子 → (#1 and) #2

Basically, proper names other than common surnames or famous places are always
accompanied by furigana.
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Newspapers do not 100% stick to the 常用漢字 kanji. They have their own style
guidelines for kanji use, and there is such a thing as the
"[新聞常用漢字表](http://ja.wikipedia.org/wiki/%E6%96%B0%E8%81%9E%E5%B8%B8%E7%94%A8%E6%BC%A2%E5%AD%97%E8%A1%A8)".

This includes:

Kanji not in the joyo treated as joyo:

> 磯（いそ） 絆（きずな） 哨（ショウ） 疹（シン） 胚（ハイ）

Kanji in the joyo treated as non-joyo:

> 虞 且 遵 但 朕 附 又

Additional non-joyo readings treated as joyo:

> 証（あか-す） 鶏（とり） 虹（コウ）

Individual newspapers then have their own adjustments to this list. Asahi
actually has the largest adjustments, and lists 66 joyo characters they treat
as non-joyo (including the seven listed above).

[This PDF](http://www.pressnet.or.jp/publication/book/pdf/shimbun_yogo.pdf)
contains the 新聞用語集 published in response to the new joyo list. It is quite
thorough, not just a list of kanji you can/can't use. For example, for 茨
there's a warning note that it should be いばら if referring to the plant, 茨 to
be used only for proper nouns (e.g. 茨城県, the reason that kanji is in the new
list anyway).
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