[Maxful]

Originally Posted by masaegu

You are asking a beginning question only a few minutes after asking an advanced question.

「Yahoo! JAPAN IDでのログインが必要です」 is completely correct and natural. How could they use an incorrect phrase when millions of people read it everyday?

The 「が」 is not even just preferred but is the only possible particle to use there. It is so because the phrase is identifying what is needed to do something. It is not talking about 「Yahoo! JAPAN IDでのログイン」.

Your "normal and short" theory has a big hole in it. How would you explain something like 「バナナが好きです。」 then? Is that not "short and normal"? It uses 「が」 because it identifies what the speaker likes. It is not talking about bananas.

To talk about bananas, use 「は」.　
「バナナは南国でとれる果物です。」
「バナナはイチゴより安い。」

Thanks for the rectification, masaegu. I confused 「Yahoo! JAPAN IDでのログインが必要です」 with sentences like 「謝る必要がないので俺は謝らない。」 and 「謝る必要がない人は田中君と小林君だ。」.