The problem is that there are Trump delegates who lied to get on his roster, and fully intended to vote for Cruz. Can they be sued for breach of contract if they don’t stay home and let an alternate take their place?
During the first ballot, delegates aren’t polled individually. A rep from each state reads out the totals awarded by that state’s rules. There’s no opportunity for disloyal votes on the first ballot.
That’s why the attention is focused on approving rules and credentials, where delegate votes aren’t bound.
I’m leaving out some details, but that’s the main idea.