There is no reinforcement cost reduction the rule simply does not apply.
Most of the units that had their cost changed simply do not follow the original rule of reinforcement cost and that includes a number of allied units.
Please do not state your personal opinions as Facts.
Sturm Pioneers
One of the biggest problems the Sturm Pioneers faced was the expensive manpower cost to reinforce. With the overall design changes to the OKW early game we wanted to make reinforcing Sturm Pioneers less penalizing on the player which will encourage their use in various builds.
•Reinforce cost reduced from 40 to 32
Original cost was 320.
320/4/2=40.
Formula checks out.
Panzergrenadiers
We are looking to minimize the heavy attrition cost of this unit. •Reinforce cost reduced by 10%
Original cost was 360.
Original reinforce cost was 45.
Formula checks out, later on squad and reinforce costs were balanced separately.
Fallschirmjäger
We are looking to minimize the heavy attrition cost of this unit. •Reinforcement cost reduced by 10%
This one is bit more tricky, because of opportunity cost of 80mp originally.
360/4/2 is 45, which is their original reinforce cost.
Formula checks out.
Obersoldaten
We are looking to minimize the heavy attrition cost of this unit •Reinforcement cost reduced by 10%
Original cost was 400.
Original reinforce cost was 50.
Formula checks out.
Infantry Sections
We are addressing the heavy attrition costs that infantry sections have. •Reinforcement cost reduced by 10% (28 per model)
280/4/2=35 which is original reinforce cost.
Formula checks out.
Now, if you still sit in disbelief about reinforce cost rules;
Grenadiers:
240/4/2=30 which is their reinforce cost since forever.
Cons:
240/6/2=20 which is their reinforce cost since forever.
Rifles:
280/5/2=28 which is their reinforce cost since forever.
Volks:
250/5/2=25 which is their reinforce cost since forever.
You can apply this to every single squad that does not spawn on field(you need to subtract opportunity cost and then it checks out, as you see on falls example) and you will get correct results, because formula you pretend does not exist actually does.
At the moment squad and reinforce cost are balanced separately, but all were introduced with a hard rule on these costs being tied together.