Mono changed the license so that it could be used in Unity to begin with. Mono was available under a copyleft license. To everyone. Including Unity. Unity couldn't/wouldn't accept it under those terms, so folks at now-Xamarin said, "We'll offer it to you under a commercial license if you won't take the copyleft offering." That's standard fare. Unity said, "Okay, deal." Mono continued to improve over the years. Unity was interested in those improvements. Mono said, "Okay. You can get those under a commercial license, too." Unity balked.
So what we're really talking about is that Unity wants the improvements that have been made to Mono, but doesn't want to pay for them.
Get this: They're still free at any point to accept Mono under the copyleft license that's available to literally everyone, but they won't accept it under any terms other than their own. That's it.