Trump doesn't have to use a teleprompter. He chooses to to make the RNC happy.
And it "makes the RNC happy" because Trump is a bumbling, incoherent buffoon who sinks in the polls everytime he goes off-script and who'd never complete a spoken sentence without going on 12 different tangents in the absence of a teleprompter.
But of course, Trump doesn't need to use teleprompters, cue cards or anything else. It's not like the dude's been relying on them for years due to his now-canceled, pretend-job-hiring show.
