I mean, I can’t speak as to Mike Johnson’s personal sense of ethics, but if he’s making a legal claim, what law does it break?
This is a party-internal process.
The Democratic Party didn’t participate in the primary election, based on the expectation that Biden would run. But as I’ve pointed out here before, there’s no legal obligation for an American political party to hold a primary election or even if they do, for that primary result to bind them. We historically – and for most of American history – didn’t hold primaries at all. Parties decided that they wanted to do them and participate, but they don’t have to do so. The Libertarian Party does participate in primaries, but under their party rules, the results are merely advisory – the party can theoretically choose someone else. The Green Party policy varies based on state: in some, they participate in primaries, and in others, they caucus, have the state party internally select the party’s candidate.
For about half of the ~150 years that the Republican Party has been around, no states anywhere held primary elections. If it’s unlawful not to go to the primaries, was the Republican Party breaking the law all that time?