Odd, there really doesn't seem to be an empty flagpole available..
Anyway, all the flags available in the editor are done using setFlagTexture (they have an init eventhandler doing it) so it should work on any of the available faction flags.
Not knowing where you use that code you posted but if you use it in a script or trigger or so it should be
this setFlagTexture "flag\usarmy.jpg"; this setPos [getPos this select 0,getPos this select 1, -2.5]
I don't know how that would have worked in A1 because double paranthesis (or what ever those are called) should not work in situation like that (unless the line is from a config event or something)..
Other thing is that depending on where exactly the flag texture is as in if it's an addon you may need to use
this setFlagTexture "\flag\usarmy.jpg"; this setPos [getPos this select 0,getPos this select 1, -2.5]
And one more possibility for this not working might be that the game doesn't allow using jpg images in this kind of situation anymore and you should use .paa instead (convert the jpg to paa using BIS TexView in the Arma2 tool suite, that is if it supports jpg)..