I don't think it's very nice to call it a stupid letter.
And good for him for giving the wts a little grief for their behavior.
Anyway, OP, I'm not sure I understand your objection in regards to these apartments. In the US, owners have a right to use their rental property for their own purposes and not renew leases for existing tenants. The tenants are aware of this when they sign the lease and move in. I'm sure it's disappointing, expensive and inconvenient for them to move, but it shouldn't come as a shock, and they're not "throwing them out on the street" really--just not renewing a lease. Unless I'm missing something?
Is it the word "luxury" that's bothering you? You might be interested in this. Sounds like some people are of the opinion that these are the opposite of luxurious.
What is the Charity Commission that you mentioned? Is it a legal entity with authority in New York State? I don't think so. The Attorney General's office has some authority over nonprofit organizations in NY but they do not get involved in complaints of this nature (people not liking what the money was spent on). They only get involved if there was a violation of law.