I don't understand how scenarios could be balanced as part of a campaign but not as free standing scenarios. If they are balanced as part of a campaign, they would need to be balanced from different starting points or "strength levels." If they are balanced for different starting points or strength levels, how could they be unbalanced when started from the historical (presumably "average") starting point or strength level?
Would it be major work to have these scenarios playable in the game menu without altering the game?
It's quite simple really. For the AI to function optimally, it requires some pretty heavily configuration. Reinforcement settings, pre-placed units, AI triggers, timing configuration, and so on. The DLC are designed as campaigns from the German perspective, so the Allied AI and starting force has been configured as such, but the German side has not because the German side has been configured to be played by a Human player.
Same reason the MP maps don't show up in the scenario list by default. There is no proper AI configuration built into any of those maps. Sure they're playable with the AI which is why you can move the scenarios if you want, but they aren't optimized for it.