A few thoughts:
1) Keeping Butler at the max will not likely cause the Bulls to over the luxury tax, which should be a little north of $80 million next year. It could be a little tight against it, and this trade would give them room for the full MLE without stressing the tax, but if the Bulls want to keep Butler and avoid the tax, it's not imperative that they make any trades to do so.
2) That said, I think this deal is a coup for the Bulls. The difference between Sully and Gibson is negligible when you're talking about your 4th big in the rotation. Gibson is overpaid for his role on the team. Essentially they clear about $4 million from next year's books, and an even more important $6 million from 2016, when they will next have cap dollars. This difference includes Stauskas's salary -- so even if you think he's deadweight it's okay, and if he becomes an actually useful player, which is still very possible, the Bulls have cleared significant salary, kept productivity at their backup power forward spot, and gained a potential big-time scorer with three years left on his rookie deal all for the cost of a pick which is top 10 protected through 2017, and becomes seconds if it doesn't convey.
3) As someone noted, the Kings would needto send out extra salary to make this work. If you believe the Bulls do this in part to clear room (logical), then that salary has to come to Boston. Sacramento has no real contracts that make sense for Boston. Ainge wants as much room as possible for free agency next summer, and Sac would need to send out about an extra $3 million in contracts that do not expire (options count as expiring) on June 30th. Really the only player who fits is McLemore, which means SAC is getting hosed. Otherwise you have to send McCallum and Sessions, which means the C's take on $3.1 million extra in salary. I'm not sure that's something Ainge does before he makes offers to free agents. Especially since that pick is no better than 11 overall.