I think, technically speaking, the GOP allowed the candidates to accept debate offers that weren’t necessarily endorsed by the party.
That said, GOP candidates need to be able to reach voters who aren’t already in a camp or strongly affiliated with a party. There isn’t anything inherently wrong with trying to convince voters from the middle and other side of the aisle to vote GOP. It’s how Reagan won - by being able to convince people to move over by persuasive arguments and good nature/humor.
“I am paying for this microphone Mr. Green” Reagan at NH debate
https://youtu.be/Rd_KaF3-Bcw?feature=shared