Each party is its own club and sets it’s own rules. While the primary elections are kinda sorta binding, the important election is at the party convention, when delegates pick a candidate, similar to the EC.

Even if Trump wins all the primaries, but something happens to render him ineligible/incapable before the convention, the convention can adopt rules that allow Trump’s delegates to vote for whoever they want. That’s ultimately why I think Haley will keep going until the bitter end. Even if she wins no states, as long as she keeps getting 30+ percent of the vote she will end up as the only viable candidate for the delegates to support if Trump goes away.