Although the reason people aren't paying attention to it is mainly laziness, jail (or gaol) and prison were synonyms for hundreds of years, and the distinction between the two was made only relatively recently (and only in America). So people not distinguishing them are preserving an older form of the language.
Incidentally, while checking the Oxford English Dictionary, I found out that the original spelling of jail was gayhole:
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The earliest meaning of prison was "the condition of forced confinement", but it seems this distinction was lost in the 1200s.