Rugby88, from what I understand the US 1st got a taste of rugby during the war when the US soldiers were hanging with the Brits.
I believe the president at the time (Roosevelt?) was against the sport as it was seen as too 'barbaric'
Hence the growth of their 'football' which afforded protection by way of helmets etc..
As you've pointed out, now you have 110+kg units with speed and strength conditioning coming at you down the pitch, the armour would almost be a necessity with some of the hits put on them!