In the US, in order to be called rum, vodka and most other spirits it must be bottled at at least 80 proof - 40% abv.
Spiced rums are just that, 'spiced rum' and as such don't have to comply with to the same standards. The difference in Federal Excise Tax is about $3 per case of 12 x 750ml bottles, which in the US is called a 9 liter case.
States charge tax on a number of critieria and for some there is no difference in the cost tax for a 750 ml bottle at 80 or 100 proof. The big savings for spirits producers is the Federal Excise Tax which every spirits producer pays on every bottle of distilled spirits sold in the US.
Ambassador of Rum
Ministry of Rum
When I dream up a better job, I'm going to take it. In the meantime, the research continues.