Shocktroops could need to buy their SMGs like Rangers, etc. There are lots of ways they could have given Soviets more interesting munitions-based upgrades.
You aren't playing since release, are you?
Shocks used to have to buy ppsh back when they were 360mp.
Then upgrade was made default and cost raised to 480, then it was lowered, because at 480 no one ever got them.