Thing is, I don't see a chance in hell that the Nets buy him out. Why would they? They still have to produce a basketball product and get fans to games, and with the way he is playing now he is one of their best players. Without any draft choices they have no incentive to tank, and anyway I can't imagine they want to turn into the 76ers.
The only incentives they could have are:
1.) money
2.) being "kind" to established veteran players.
Both of those require Joe Johnson actively asking to be bought out, if he is making the request, and say, is willing to take the buyout as if he had a $20M salary this year, rather than a $24M salary (just as an example), it would save the Nets $4-5M. They'd save about $1M more, because whatever he gets paid by another team (probably veteran minimum) gets subtracted from what the Nets owe him.
Also, if a vet who has put in his time in this league, and is in the last year of his deal, asks for a buyout from a team going nowhere, so he can chase a ring -- then you have to be a jerk to say no. And no team wants to look like a jerk to well respected vets.
But you're basically right - the Nets have no real incentive to buy out Joe Johnson, unless Joe Johnson provides them the incentive (by asking them to do it).
Another fair point, is that Johnson might not want to do it -- he'd probably give up $3-4M in salary, in order to have to play 2-3 more months of basketball. Lots of players would give that up in order to chase a ring, but not necessarily everybody.