With BA its 1 in WT, 2 in WT+, 3 in club and 5 in first. The one bag rule came in October I think. You can actual check in as many as you like you just have to pay for any over your allowance.
It has nothing to do with US regulations its just what the market is.
If you are an executive club of a certain level then you can have more.
Sorry buts thats the way it is.
P.s. its not just the wieght there is also a size limitation. All can be found on the BA web site.