its not a free buff its a weapon upgrade path. think about it, all the axis have some sort of specialized mainline infantry they can get with tier buildings (pgren and obers) while all the USF have is the rifles. the Lt. and captain would not get buffs to the unit itself, simply its toolkit. These upgrades would most likely be very pricey as well. I feel a small change like this will give variety with USF play so it isnt just 5x mid range rifle squads running around everywhere. I would also say they are not that unique, they are simply rifle squads with smoke but missing other items such as grenades or at nades. Sure the Captain can speed up buildings but that is about as far as his uniqueness goes.
But its not a weapon upgrade at all. To be honest the picture you have of axis infantry is wrong, because one trades versatility and generalist capabilities (both sinergyse too) and the other requires a mixture of units to fullfill the same roles. Riflemen can be massed without the same risk as grens/pgren/volks/obers.
Imagine it like this: Riflemen are half penal squad + quarter obers + quarter grenadiers and T0 mainline inf. Riflemen AI with BARs or M1919 (i know is rarely used though) is competent vs equal numbers of axis infantry, AT potential against LVs and light tanks is also good enough to solve their threat. Both desitions are reactive, wich means you can get the upgrades AFTER the enemy shows their units. That pricey.
If you were playis axis, as an example and you massed volks but no raketen, when a T70 or a stuart appears you are simply forced to back up, a minor loss but you also would need to allocate manpower to get AT, this kind of reaction is slower when compared to USF, it has to pay a less expensive, fuel dependent, side tech and after that you slap in munitions but no manpower, any riflemen squad could receive the weapon even when they need to go back to base to get the zooks.
If you were to say,
but pgren wouldnt do the same? Yes, you would be correct, but pgrens are also more expensive than riflemen and pzsheks comes only in pairs, you are forced to get both, therefore a single pgren squad becomes AT exclusive, until you get another one to make up the AI loss. Pgrens are less versatile, hence more effective for balance purposes. Its also worth mentioning that this kind of reactions tends to leave OST weaker in the AI or the manpower bleed aspect, that can be exploited when playing USF with howitzers or shermans
Every unit toolkit is considered in the unit cost. If you were to add a new toolkit and try to keep things balanced, you should increase LT or CPT cost proportionally.
I would say that CPT is a bit off in the mainlines and since the USF tech rework its often used like this. I would give to the LT the CPT recall ability and change the CPT to have either a stationary aura effect (that should be toggled), or maybe a new ability to be able use rear echelons to transport weapon upgrades to its position.