No it's not. Japanese cellphones probably have had Japanese-English dictionaries in them even before smartphones took hold here. I saw them being used to look up English words all the time.
What Japanese are attached to is the 12-key flip phone form factor. So much so that Sharp released a 12-key flip phone running Android a year or two ago. My guess is that the dictionaries fill a high-end niche that cellphones historically could not: when you need a more extensive dictionary, have to look up a lot of words or translate entire sentences, and need to type quickly. As smartphones (iPhone, Android) become dominant, I think those dictionaries will disappear.
What Japanese are attached to is the 12-key flip phone form factor. So much so that Sharp released a 12-key flip phone running Android a year or two ago. My guess is that the dictionaries fill a high-end niche that cellphones historically could not: when you need a more extensive dictionary, have to look up a lot of words or translate entire sentences, and need to type quickly. As smartphones (iPhone, Android) become dominant, I think those dictionaries will disappear.