To answer your questions:
> Whats wrong with running ads for condos in a whites-only resort community?
- profiting from racists
- ghetto creation
- perpetuating harmful discrimination
- assisting in the longevity of a community which exacerbates the prior 3 issues
> If their internal numbers show that this community has a really high chance of conversion, why should they target something else on a finite ad budget?
They shouldn't. But also it shouldn't be available to them, by law.
> Can't a company decide who it wants to sell to?
In the general case, yes, but since the civil rights movement, you cannot refuse goods or services to someone based on race. That includes housing. I was under the impression this was widely known.