The people pissed about this are probably the same people who think there is no difference between Hillary and a Republican.
More debates is not good for the party. A socialist of all people should understand that the cap is for the common good.
I don't disagree with your overall point, but how is waiting for the general election more helpful than hashing out policies and positions in the primary?
Because it limits the ways in which interests groups can manipulate the process.
I can't think of a good democrat example right now, but let me illustrate with a Republican one. Let's say some anti-immigration group had good clout in Iowa, and wanted to host a debate. They networked and fundraised and invited all the Republican candidates to speak. Some of them would probably jump right on it. Some might be cautious. So then, you've got Marco Rubio or one of the other immigration "moderates" not wanting to wander into a landmine, but not wanting the other candidates to get the airtime and publicity that he would not. Maybe that debate airs before an official debate, and perhaps more people tune into that one than the official one. If you are the RNC and you want your party to win, you might be nervous about these groups that are unaffiliated with you using their powers to divide the candidates and manipulate outcomes by pressuring candidates to make unofficial appearances, asking loaded or biased questions, and talk about issues of importance to them. News organizations and the various debate moderators may have some biases, but they aren't "interest groups" in quite the same way as the organizations that might want to host the unofficial debates.
Yes, the public has a right to hear all this information to decide between the candidates. Sure, it's better for the public to have as much info as possible, and because debates are cheap and efficient ways to do that, I can see the argument that more debates are good as a general matter.
But from the party standpoint, they have a huge interest in controlling where and how it is disseminated to ensure the party wins. Limiting debates to official ones only are a good way to do that.