Nobody said that they "couldn't" go, it is that they see no point. Like the D party assuming their vote, the R party assumes as well. Therefore they have no power with either in actuality.If a candidate can't go to the function because he feels the sponser is partisan, then how is he going to be President for anyone other than the people who voted for him? That's more of what we have now.
They are speaking to the community at large, especially to the black community, who aren't necessarily at the event, but who are watching.
Ironically, it is the R party who have appointed people with darker pigmentation to positions of power far higher than any other party. If it was for actual advancement, instead of labelling them as "too white" or "oreos" as I have seen this group would have a different view of the party as a whole.