The purpose of a primary is to help the party pick a nominee that has a better chance of winning the general, which is, in turn, the purpose of a political party. The mechanism of binding primaries was set up by party leaders after some bad choices (especially in the 1968 Democratic Convention). This time, the prospective candidates decided that a blitz primary wouldn't serve its purpose. If the voters punish them for this decision, then it will have proved a bad one, but it's neither unprecedented nor undemocratic.