In my opinion there was not much risk involved in this specific case.
In the trade Kings received c. $3m in cash considerations, which is exactly what was necessary for the buyout.
It was clear to everyone that the Kings are not retaining Joe Johnson.
Well, it's not as if every time a non-contending team deals for an expiring (usually veteran) contract at the deadline that they buy the contract out. It's pretty common, and you can anticipate it, but you can't necessarily rely on it.
That said, I doubt it was clear to
anyone prior to a few hours before the deadline that Joe Johnson was going to be dealt to the Kings at all. And with Utah about 2 games down of a playoff spot I'm not sure that, had they not traded him, the Jazz would have cut him. The original question was: Why didn't the Rockets just straight up trade for Joe Johnson? The answer is that they didn't really want Joe Johnson (and his contract, or any contract like his) that badly. They wanted a proven veteran on the cheap for their stretch run. They waited until after the deadline for the carnage to fall out and figured that being a frontrunner they'd have their pick of the buyout crowd. Then they took their pick, and their pick turned out to be Johnson. Had you asked Daryl Morey on Wednesday, 24 hours before the deadline, who his biggest buyout target was, there's probably a good chance he doesn't say Joe Johnson. But he didn't know how everything was going to fall out, and once it did fall out, his decision was clearer.