That is because Concentration acts at the level of Gloranthan Physics (or Metaphysics), while Homeland-specific Common Magic come from the level of the individual society. The two methods are almost orthogonal, at best.
Also, remember that Concentration is a rare thing, only used by Hero wannabees (like PCs), so the advantages and disadvantages seldom matter, except during the heroic actions at which we RW players spend far too much time at to properly model Gloranthan societies.
At the start of the Hero Wars, I expect that average non-Lunar Orlanthis are just as disturbed as you that their prejudices against the "weird" magics of their worst enemy, the Evil Emperor (at least, he is ONE of the worst enemies), are not supported by the way that the (Other)world works, and the Yelmic nobility just as shocked that the converse is also true. By the time that Argrath does the Net Ritual and kills most of the Gods, I expect that this knowledge is far too widely known for problems with Common Magic to matter. And then, the magics all change and render this knowledge as irrelevent, if facinating, as the Forbidden Secret of the God Learners.
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