There are plenty of problems here, not least being that it is downright illogical to assume that a person can simply will his or her muscles to stop cramping, or that it is physically possible to play through cramps that are fundamentally incapacitating. (And keep in mind the context: 33° Celsius heat in which James had already played for 33 minutes.)
Let us assume (incorrectly) that it would have been possible for James to play on, so long as he was willing to endure pain. According to this line of thinking, shared by Jackson and countless others on the internet, James violated one of the first tenets of masculinity: stoicism. So-called 'real-men' are supposed to endure any amount of pain without admitting that it bothers them. Pain is never to keep a 'real-man' from meeting his objectives. This, of course, is a line of thinking that distinguishes so-called 'manly' men from gay men and, of course, from women, who are traditionally (and erroneously) thought to be unable to persevere through any sort of sensorial discomfort.
Leaving aside the fact that this contrast is completely false, let us examine it on its own terms. Is subjecting oneself to extreme pain an inherently virtuous act? Is this an attribute we should, as a society, be privileging in anyone? Certainly, it is impressive to watch another human being cope with arduous circumstances, particularly when it is in the service of a cause we deem to be significant (and, yes, professional sport has been determined to be such a cause by a not-insubstantial segment of the world's population). But what is the impact of that pain on the person who endures it?
For professional athletes, enduring pain is about pushing the body through injuries, often with debilitating long-term effects. It also means conditioning oneself to ignore the needs and desires of the body and the self. This is true both physically and emotionally. In order to embody the toughness demanded by masculinity, athletes must treat themselves as if they are impervious to feeling. Any sign of emotional vulnerability becomes a betrayal of that veneer, to the point that the stoical performance can actually be internalized as the true self. This process of conditioning has the potential to foreclose the possibility of intimacy in relationships with others, as any hint of vulnerability threatens to collapse the entire construct of masculine toughness. It produces men who struggle to nurture and care for the other people in their lives and who struggle to find emotional fulfillment and satisfaction for themselves.
So, what does this all have to do with #Lebroning? Well, professional athletes are often referred to as role models. This is not so much because they choose to model behaviour or because they are ideally suited for the role. Rather, it is because they operate under an incredibly bright spotlight, and what they do is observed by millions of admiring people. When athletes perform masculinity, they simultaneously instruct countless other boys and men to do the same. Thus, when Mark Jackson says during the NBA Finals that great players play through pain, countless viewers learn a lesson about what it means to be a man. Likewise, as silly as it may seem, the #Lebroning meme drives home a very serious message about how men are permitted to act. By ridiculing someone for experiencing pain and vulnerability, we contribute to the production of a society in which stoicism and toughness are prized above all else. That is a society in which we all suffer.