Nah, don't really see the point in this.
The US is already enough of a pain in the ass to conquer as it is considering its geographic position. I don't see what warrants the US getting special mechanics for this beyond what other countries get. If you think it's because of the stereotype of the US as being gun crazed, that was actually much less true in the 1930's than it is today. Additionally a lot of other countries that have banned or heavily restricted firearms today had much less stringent laws and firearms were much more prolific. Norway comes to mind.. in fact, gun club members in Norway actually banded together and held off a German attack to give the King time to escape.
The idea for the Spirit (and the actual name of the spirit) comes from an apocryphal quote attributed to a Japanese admiral regarding the chances of a Japanese invasion of the mainland US. I think it might even still be one of the quotes used in the HoI IV loading screen. I know that the quote is bogus, and that the US wasn't necessarily more gun crazed than some of its contemporaries, but it
does line up with how Americans at the time
saw themselves. Gun culture was just beginning to hit its early stride in the US in this time period and since this is a "
national spirit" we're talking about it wouldn't be strange to put it there in my opinion.
Also I feel like it would balance out early attempts to conquer the US, since doing so totally skews the balance of the game in the long run.