If a unit is of a unit class that is capable of indirect fire, than all weapons that qualify will fire.
Indirect fire is a capability of the
unit class and not the weapon.
Some OOBs have these as direct fire units only, but some OOB designers have made them indirect fire classes. what the correct approach for the SP games is - who knows
However - simply making them say CS-tank class has problems if they were say used in a scenario as a pre-game bombardment.
We will have a look into the matter.
Cheers
Andy