Another Spurs fan here, and a long time NBA fan. ChickenS move by DeAndre.
This is not a typical labor market where you have a large number of roughly similar job candidates available year round and you can pay them whatever you want. This is a very peculiar talent acquisition process with labyrinthine rules regarding how and when you can commit money to a small number of job candidates with highly differing skill sets.
The NBA draft occurs, wherein teams attempt to acquire players to fill needs without knowing beforehand what they will get. The moratorium specifically exists so that teams have a few days after the draft to evaluate what happened in the draft, speak to all their targets, negotiate contracts, and model out scenarios based on who they can afford, what will make a good team, and how they can get the money together. There are contract max and min limits, salary caps and mins, lux taxes, bird rights, trade exceptions, cap holds, the room exception, biannual exception, Rose rule, and any number of other things I've forgotten to mention.
The way this has always played out in the past is that the verbal commitment, once given, is viewed to be binding. Once the verbal commitment is made, the player who makes the commitment knows that his future team will use that to (1) decide what other players they need to fill in around him, (2) evaluate the roster cap situation and scenarios, and (3) target and recruit other players to fill those holes, including using the news of the first player's commitment as a tool in recruitment.
DeAndre, as a young max player, knows that the Mavericks committed a lot of money to him and were building around him. By the time he went back on his word, the other high profile free agents had almost all committed to other teams; what is left is dregs. He put the Mavs in a situation that they probably can not recover from for at least a year and maybe longer. Not because of the quality of their plan, but because of the rules of NBA free agency.
If you're not sure what to do, you don't commit. If you commit, you stand by your word.