Er....because where they come from, ie Australia, NZ, England, Ireland, Wales, Scotland, France, Sth Africa etc etc, American football isn't exactly a hugely popular sport, and playing pro Gridiron would make them about a one ten thousandth or so of what they get playing pro League?
And if you seriously think that pro gridiron players are that big, hard and fast, what are they doing encasing themselves in pads and helmets? they skeered or summink?
This is just ridiculous, both sports at the professional level are exactly that, proffessional, I am not saying either one or the other is 'better', just pointing out that US basketballers arent the supermen you say they are ( it would take some rather serious individuals to take on a pro league team AND the crowd) and if you really think the Australian Rugby League side(or any other representative side) are girlymen or something similar, it is blatantly obvious you have never watched a game.
Weather or not the Americans get into the whole thing is beside the point, the game is being played there now, the US has an international team, and regardless of weather they do any good or not, just having a go says a whole lot about them.
Personally, I hope the Yanks give the poms and Kiwis a hiding

PS what is the US side called? Aust=Kangaroos, NZ=Kiwis, England=Lions US= ? Eagles??