When STM was introduced on the VXRs all devices were using DoCSIS 1.0 Profiles and therefore QoS Profiles.
STM on QoS Profiles applied the penalty to both directions as default.
Once they started to move to D1.1/D2/D3 they introduced Service Classes
STM with Service Class is uni-directional by design.
So the fact that it was bi-directional was only because there was no other way of implementing.
As is now the case there is no way to run STM with Services Class other than uni-directional - So VM don't have a choice
