Due

The reason why the upgrade  is locked at vet 1 is beacuse for balance purposes. The whole AT squad cost 180 manpower (Comparing to other factions AT squads they are really cheap), even if they die you can replace  them really fast and cheap. Losing for example an American AT squad (280MP) could be an dramatic loss ( depends on manpower income). And I want to make kinda choice/reward mechanic in this, you can get at vet1 ally equipment that is pretty stable or you can fight to get   potent panzershreks.

I personally think that getting vet 1 on soviet AT squad is easy, just dont rely on PTRS but on AT grenade.If you see any kind of tank, charge it, use AT nade and retreat.

oh, and Soviet AT squad is unlocked when, T1 and T2 are unlocked.(Only for motorized i think)

But for British,SS,American and Wermach when the T3 is unlocked