Can a POA decide who visits and who doesn’t?

This is usually because the visitor causes agitation and/or behavioural difficulties. Whether you are purposely stirring things up or not doesn't really matter, but since the POA refuses to discuss it with you I have a feeling you have had discussions in the past about what lead to this decision.

Yes, it is legal. I would throw myself upon his mercy with a letter that you would love to continue to visit, that you are saddened, and that you would do ANYTHING to be able to visit. You can agree to visitation with a family member accompanying you. You certainly are free to question the son as to why this has been done; he may have reasons. He is free not to answer you. My GUESS (and it is purely a guess) is that the son is quite afraid now that his loved one has gone into memory care that she will begin to pass out money to any and everyone she sees. Believe it or not this happens, and I just saw it happen with a dying friend. As POA he should already have taken steps to insure that no such thing can happen. But guesswork gets us nowhere, so I would write a simple letter asking if you can please visit, even if accompanied. And I would send notes and cards and pictures and flowers and goodies on her diet to your good friend.

Yes it is legal.
It is possible that after your visits she would become upset. (unbeknownst to you)
If this is the case and it would take the staff a while to calm her down they may have said something.
It is also possible that she has said she does not want visitors.
It is possible that she has declined to the point where she does not recognize anyone.
You can..
Write her a letter.
You can send flowers.
As difficult as it is accept the fact that this woman is no longer part of your life.