If one plays the hypothetical that Howard and Smith want to come to Boston, then:
1. The Lakers dilemma... 72-79 million already on salaries. They don't have the means to acquire people because they don't have any assets to make worthwhile trades. They have a depleted team with lots of free-agents.
Why would they say no to a package that includes some young promising assets, some draft picks, and players who can actually contribute this and maybe a couple of more years before Kobe retires? When the alternative is to let go Howard for nothing.
2. The Hawks dilemma... a ton of cap space, but you barely have any players signed this coming season, so that cap space is quite deceiving. Further more, this year there aren't any worthwhile free-agents to acquire to use that cap space on. The only ones are Smith (who you have no interest in returning), Howard (who has now shown he wants to go to Boston and maybe has no interest in going to Atlanta), Al Jefferson, among a few others. Not a real deep free-agency class to spend money on, and since you have barely any player signed, no real attraction.
Why would you say no to say getting Humphries (an expiring contract, who has no effect on your future cap space) and some picks for Smith. Further more, you can move Humpries in a subsequent trade, probably at the trade deadline and maybe acquire something of value for him, maybe a pick, or just let him expire, doesn't matter much.
You can do all this, and still have room to spare to go after a max contract type of player if there's still available, or go after a few lower salary types.
So, why would you say no?
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That's the way I'm seeing it from their perspective, and I'm having a hard time finding a way to say no to these sorts of packages when there aren't that many alternatives to get value for them... the free-agent players have most of the leverage.