Serena markets herself as a serious individual that stands up for social justice issues.
The seeming double standard here is one that Serena herself has created. She herself has occupied a position where she is required to stand out as an exemplary sports athlete, rather than a sore loser and a temper-tantrum throwing dickhead like McEnroe.
If she had built her "brand" as another McEnroe, sure, people would probably look the other way more than they have done in this scenario. But then those same people would probably also give way less credibility to what she says or does, out of the court. McEnroe's behaviour followed him forever, and wherever he went, he was always the "bad boy of tennis", nothing more. His otherwise successful career is mostly brought up because of his incidents on court, not because of all the wins he had. That is what people remember.
Lets face it, Serena doesn't want to be another McEnroe, because it would amount to her just being seen as a quirky individual with no great credibility, who is renowned by her bad behaviour on court. She is better off with people treating her recent outburst as an exception, rather than the rule.