Why get these units in the first place then? they cannot be potent recon assets if they need to be in cover. thats a false statement.
Stationary vision for infantry has always been bugged back in time when Riflemen had it in I-dont-remember-which-doctrine. O maybe it was also because they needed to be in heavy cover but from memories it was unusable.
I wouldn't say they can't. Spotting scopes work just fine that way and are very much viable, though these are of course vehicle-borne and, thus, easier to utilize due to the higher mobility compared to infantry. Still, if positioned properly stationary recon units can provide great vision (especially those with passive cloak), just not as offensively as PF or JLI are able to do now.
IIRC Volks still get some passive vision increase with vet when in cover (at least according to the veterancy guide), similar to what said rifles used to have. Not sure if this is/was somehow bugged but obviously any ability similar to that would have to be implemented to work properly.
But in reality I just pointed out the inconsistency in their decision making, same as the damage reduction removed from Ranger and then 3/6 months later given to Gren. Too good for USF but never enough for some other factions. Guess what, people don't use ranger anymore they get shredded by tanks, Path and Scott/jackson all the way now.
I get you and this does sound like double standards... at least kind of. Still, though I'm not 100% sure I'd bet Rangers got compensated by having better RA to make up for the lost DR against small arms (just like Grens, which actually even lost some EHP in the inverse RA-to-DR trade). Now that obviously doesn't help much against tanks as you said, but Rangers also gained access to the beefed-up super zooks in return. High DR on a 5-man squad able to wield 3 of those would be way too much.