Differentiation between Company Sets and Factory Sets...please chime in....

My Registry has a mix of Company Sets, Team Sets, and Player Sets.
Why does PSA only allow cards that have been graded to be added to player sets, but on a good many of the Company Sets (esp modern), PSA will list all cards in the set, even though there may be one or several that have never been graded? Is this PSA's way of "forcing" you to submit cards to complete company sets?
Thought and theories are appreciated....
Why does PSA only allow cards that have been graded to be added to player sets, but on a good many of the Company Sets (esp modern), PSA will list all cards in the set, even though there may be one or several that have never been graded? Is this PSA's way of "forcing" you to submit cards to complete company sets?
Thought and theories are appreciated....
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