Wow. I can assure you it's not as simple as that. Myself as a QB who (without sounding like, well, I don't know what) was a little like Newton (in that I was fast and agile, but bigger than most defenders coming at me) I used to love if someone came at me fast. It made it much harder if they stopped and set up, but then you would tend to try and run over them instead, but more often than not, even if you knock them down, you're probably going down too.
I don't watch a lot of rugby, but I thought most of their tackling was kind of side on. They didn't often hit people head on surely?
That's not to say there wasn't some bad tackling, Drew Brees putting a spin move on two Falcons was a great example, but errors are usually down to bad angles more than anything. That's the problem when you're head on with someone, you don't have an angle. (I played a lot of FS too, and hated open field tackling myself)