If this is actually intended, it's the worst implementation of the feature that could possibly be done.
You've got a pop counter, which is the hard cap for every single team in the game, including USF. However, IF the USF player gets out of the vehicle, it lowers the pop-count (not explained anywhere in-game), allowing for more units to be made. It's then possible to get back into the vehicle, exceeding the pop-cap (also not explained in-game), resulting in the UI for the pop-cap to turn RED (i.e. bad), as well as display a value over 100%. All of this is without any consequence (which the red UI would suggest).
If this was actually intended, it would be explained either in a tutorial or via the UI, and the pop-cap wouldn't simply turn red. Further more, there would be some downside for exceeding the pop cap, as a trade-off of being able to exceed it in the first place.
No, my bet is that the feature was added, and this was an unintended consequence to which they don't really have a good solution to.
USF crews are intended designs in the game. I'm not sure if you're aware of this, but there is no multiplayer tutorial for USF. Maybe, by some slim chance, there's some info in Ardennes Assault, but other than explaining the basic mechanics of the game, there are no tutorials for each faction, unfortunately. That's something that you've just got to deal with.
There is a consequence when you go over popcap. For every 1 popcap, you lose 1.5 manpower, I believe. If you're at 150 popcap (god forbid), your mp income is 75. Those numbers could be wrong, but the mp income relative to popcap stays the same no matter what popcap you're at. It's a linear equation. m=-1.5p + 300.
Leave it, but implement a MP and Fuel penalty for every 10 over the pop cap. It is just dumb when they can consistently reinforce 8 squads of RE while having 8 M10s on the field and fuel for replacements at the ready in a few minutes.
There's a mp penalty, as explained above.
If it's dumb that your opponent can replace tanks rapidly and reinforce squads, you've done something wrong. You've lost. You've been outplayed.
Sure, it might seem easier, the game may go on longer if your opponent was at 120 popcap, but they had the resources to spend on that shit and they're maintaining enough mp to keep that shit on the field and deny you the ability to recapture territory. You. Have. Lost.
In 4s, it's definitely a different story.
With Calliopes/Priests, it's possible to just popcap abuse. I do that but only if I have a solid fighting force first. I have 2 jacksons, LMG rifles, maybe an AT gun. If your opponent has 12 calliopes and a couple of fresh rifles, they're not a good player or you and your team suck enough that your opponent can have no frontline armor or infantry and still kick your ass.