I asked this same question when I got into the games 6 years ago. The consistant answer I received was that rivers were used as a means for carrying logs down stream to logging mills. The end-over-end method of caber tossing ensured that the bulk end of the log (being heaved from the bank) would place the log mid stream without worry of bottle necking against the shore line. Seemed a little far fetched but it is the only explanation I have heard, and from way too many sources for me to not believe that there is at least some element of truth to it.