Vehicles with the default Autonomous AI Units, is not auto-rearmed .
For the SAM-s in game, without workaround it would be a big problem . Because 1/+3 Rheas can be emptied by a single FA-181, with no harm, in ~2 minutes, even without spending all flares . And essentially, no players want or should waste their gameplay time to monitor AA .
**Workaround** : Delete the AI, move some infantry into the vehicle .