From what I could find doing some (brief) research, this is a case in which the difference breaks down nearly perfectly along generational lines. Those ages thirty-five and under overwhelmingly prefer "on" and those younger prefer "by"[1]. Prepositional choice has always seemed a bit arbitrary to me, and the fact that there are dialectical differences reenforces that belief[2].
To suggest that this contributes to a "generation holding the less weel read attitude that grammar doesn't matter" strikes me as a bit misguided.
[1] http://www.inst.at/trans/16Nr/01_4/barratt16.htm [2] http://en.wikipedia.org/wiki/American_and_British_English_di...