It's by invitation. Besides there will be many donors there who want to hear different positions. It gives everybody a chance to rub shoulders with the whole spectrum of candidates and donors.
Then to me it was done out of courtesy with regard to his two former President relatives by the whatever CPAC organizing committee. My guess is they felt it an unpleasant task, but required given the provenance of his family.
Regardless, it seems to me if this walkout occurs, the organizing committee would have overstepped their bounds and pissed off the rank and file members. We shall see what happens.
I don’t really give a crap, frankly. If he is THE nominee, then I just stay home on Election Day.