This is another example of you not knowing all the facts before you speak. I already know about the syndication rights. What you don't know is, both the Big Ten and SEC had to buy back syndication packages before they created their networks. For example, this is an article about ESPN acquiring the rights for the SECN:
ESPN is in the process of regaining its syndicated rights from Comcast SportsNet and Fox Sports Net. Both RSNs currently buy SEC games from ESPN.
http://www.sportsbusinessdaily.com/Journal/Issues/2013/04/15/Media/SEC.aspx
As you see, Fox had a syndication deal for SEC games. So, I'll turn your question around on you. Why did Fox sell back SEC games to their primary competitor? In fact, Fox sold back SEC games to their competitor, specifically so their competitor could create a channel to compete with Fox's BTN. So why would they do that? When you answer those questions, you will know why they will sell back the ACC games.
Raycom is an easy answer. They will go out of business either way. If Raycom plays hardball, and prevents the ACC from getting a network, then the ACC will just ask ESPN to use another syndicator when the Raycom contract runs out. ESPN will do it, because they have no loyalty to Raycom. They only used Raycom because the ACC specifically asked them to. ESPN would just as easily syndicate with Fox or Comcast, just like they used to do with the SEC