My grandparents hit their children because they knew no better.
For the same reason, they smoked.
Nowadays we are more informed, we are encouraged to make informed choices. We have the benefit of research and hindsight.
I can't believe that anyone even tries to justify hitting their child. I have been a parent for 12 years, so I feel I have come across pretty much every scenario with either my children or others in which the decision to hit or deal with it in a rational, controlled manner has occurred.
And to all you 'empty vessels' out there who think 'it never did me any harm', I'll direct you to a bit of Larkin, 
They fuck you up, your mum and dad.
They may not mean to, but they do.
They fill you with the faults they had
And add some extra, just for you.
But they were fucked up in their turn
By fools in old-style hats and coats,
Who half the time were soppy-stern
And half at one another's throats.
Man hands on misery to man.
It deepens like a coastal shelf.
Get out as early as you can,
And don't have any kids yourself.