Does it have to do with sign and trade restrictions for apron teams?
There don't appear to be any rules regarding teams over the apron
sending players in a sign-and-trade. Teams who
acquire a player in a sign-and-trade are hard-capped at the first apron. The relevant language from the CBA is as follows:
Team acquires a player pursuant to a Contract entered into in accordance with Section 8(e)(1) below
Section 8(e)(1) is the section dealing with sign-and-trades; it does not include any restrictions based on the apron. (Furthermore, the Clippers do have a path to not go over the first apron anyway by letting Harden walk, perhaps that's unlikely but in principle it could happen.)
Now, the Warriors specifically would have a difficult time doing a sign-and-trade for George, because once they did they would be hardcapped and unable to go over the first apron, and they're right at the first apron as it is. The Kings actually have some breathing room, so I don't see why they couldn't find a way, except for the fact that the Clippers would probably (probably?) rather let George walk than take back a Huerter/Barnes package.