You can believe what you want but that loyalty stuff is only ever seen in the movies- that's why people review contracts and make sure that every single clause is important.
At the time DJ had a verbal understanding with the Mavs, he must have sincerely thought that he was making the right decision but a few days later he came to the realisation that he wasnt - signing for the Mavs after this realisation would have been doing a disservice to himself and any self respecting human being would never do that to themselves.
Relationships are important but understand that you are not dealing with friends or family.
I have been involved in a lot of deal negotiations as part of my current and previous job: best believe that if circumstances or people's feelings change, one party or both will and do pull out of the deal despite the commitments that they have made. It hapens all the damn time and everywhere.
Again, regarding the NBA, teams have no loyalty towards the players. They are more like pawns vs. people with feelings - players get discussed without their knowledge and sometimes find out that they have been traded the same time the whole world does- on the radio, TV or twitter. Within a few hours, they are expected to have ditched their fam and homes to move to some other place they never thought about living in. Cuban knows this too well and he should take this L like a man. Props to DJ for looking after himself and not signing on to a deal that he realised he didnt want just because someone's feelings on the Coli will get hurt. Besides, the Mavs werent completely in the dark and surely knew that the deal wasnt done before the signing - a lot can change with daylight and if they somehow forgot this, they were being stupid