I wonder what the reaction would be if Dak had said "I'm in it for the money" instead.
Probably the exact same reaction as the current one. Nothing is stopping anyone from speaking with the media. That is especially true for professional athletes.
However, pre-evaluating what will be said publicly is something every person should do when a mic is thrust into their face. The public judges what comes out of peoples' mouths. And how they shall interpret what is said will be processed through the lenses of their own life experience..
In this particular instance, there are a tens of millions potential listeners and readers, who have miniscule annual earnings by comparison to what Dak Prescott has and could yet still be compensated. Most of these people will take what Kyle Brandt and he both said and run with it negatively for that reason.
Prescott receives more than enough backlash for his fame and fortune. The NFL is a business and he is getting his bank according to how much his employer wants. Most of the grief is misdirected at him. He is not paying himself. That said, stating wealth is not his motivation can and has been interpreted as shallow. He appears to be flaunting his good fortune. That will never be a good thing--regardless how he may actually feel his words should mean.