Says who? There's absolutely no rule to that effect, and when Trump recently demanded that the RNC "shouldn't allow" Kasich to stay in the race, he must have been smoking crack because they don't have the power to stop that.
Everyone with half a brain knew that this primary field was overly crowded with 17 candidates. That alone should have let everyone know that this primary season might look a lot different than most.
If Trump was a lock to get the 1237, the other candidates would have dropped out. But since he isn't, his competition is staying in there.
If anyone didn't like those rules, they shouldn't have gotten into the game in the first place.
The purpose of primaries is to avoid convention battles, not have them!
Candidates drop out when they see they cannot realistically win the nomination outright.
The RNC is allowing Kasich to stay in (by not pressuring him to leave) because they want an Est. guy in the race, all of the others have been knocked out!
Why did Bush and Rubio drop out?
According to your logic they should have stayed in and just stopped Trump from getting the majority and then fought it out at the convention!
Kaisch has won only one State, his own.
Rubio won more delegates then Kasich has, or close to it!
This is simply a stop Trump at all costs, ignoring how primaries are suppose to work, not block the front runner and throw it back to the convention.
Says who? He already answered even before you asked the question, in the post you quoted: the spirit behind the rules.
And I add: the tradition. When candidates were eliminated mathematically from the first ballot, they dropped out to not make a mess of a convention which the voters won’t put up with. Romney dropped out to let McCain cleanly get the formal number. But now Kasich is blocking Trump preparing the way for an illegitimate Paul Ryan or most likely Romney nomination.
And I add: the voters. You mess up the convention and steal it from the candidate with easily the most assigned delegates on the first ballot, the voters won’t show up after seeing such travesty.