Joining the allied side was already spurious enough, but this is different in that the countries that the US fights can harm it significantly more than Germany can. The CP can offer the US Canada but at this point, Canada is no longer the threat to US interests that it once was and is increasingly in its orbit antway. Subduing any of it gains little, comes at huge short term cost and makes Britain into an enemy (which is a lot more dangerous to the US than Germany). The US has about as much reason to annex Canada as Germany does to annex Switzerland. Aside from Canada, US participation only results in a chance to lose a ton of money, trade and lives. The balance of power in Europe is largely irrelevant as far as most Americans are concerned.
The only reason that the US goes to war on Germany's side is if Britain has so throughly screwed with uncle Sam in the recent past that Americans believe that this offers an opportunity to knock it down a peg indefinitely. Britain,which has everything to lose and nothing to gain by doing that, is unlikely to be that stupid.