I suspect that one of the issues at Shore is that the intake is almost exclusively sons and grandsons of old boys, so there's a bit of a closed gene pool and a very academically focussed one. Also I suspect that while much of their demographic might be rugby supporters, they don't all seem to be rugby participants. I'd reckon you'd find a lot of the soccer playing boys at Wallabies and Waratahs matches, but not many at Sydney FC or WSW matches.
Maybe mumsie doesn't like the idea of little Hamish being set upon by ruffians on the rugby field?
Shore's priorities don't lie with rugby, merely rowing. Though, I am not sure how many of those boys would go and watch rowing other than HOTR. If they prepared their rowers the way that they prepare their rugby teams, they wouldn't win a single race. If it was the gene pool they wouldn't win a single race, either.