Tanking is supposed to be illegal, but the NBA doesn’t enforce the rule. During prohibition, law enforcement knew that speakeasies existed and didn’t do anything. When Mavericks owner Mark Cuban said that his team should lose games, Adam Silver, the NBA commissioner, fined him $600,000. That may seem like a lot, but Cuban has a projected net worth of $3.7 billion.
If you want to know why the NBA does not punish teams for breaking the no-tanking rule, you must know why the rule was created. In other leagues, like the NFL, there are powerhouses (like the Eagles) and teams like the Browns (0-16 last season). If the Browns play the Eagles, it will most likely not be competitive. The reason that the NBA established the no-tanking rule is that they hope that more fans come to games, thinking it will be too über-competitive. Again, when more fans come, the owner gets more money.