Anyone who is a boss, who is a coach, who is a leader understands how CRITICAL respect, subordination and obedience is to running an operation, a team, a business, etc.
INSUBORDINATION, DISRESPECT OF AUTHORITY and REFUSAL TO FULFILL ONE'S RESPONSIBILITIES are sins that are unforgiveable to an organization, team, business and sins that undermine the very fabric of a group that relies on teamwork and cooperation for success.
As you said, drug use, off-field issues are personal problems. To the degree that they impact the team is the degree that you punish them. But insubordination, disrespect of authority and refusal to obey the instructions and demands of your boss will get you fired/dismissed beyond those other sins because it undermines the very concept of the authority-submission principle.
And THAT is why it's viewed differently and why Hardy - if reports are true, and I believe that they are - is no longer with the Cowboys.
No matter what one thinks about Jason Garrett, he is the coach. And no matter if Hardy thinks he knows more about football than Garrett or any other coach, he is (well, was) an employee, a player for the Cowboys. And as such, he had men in authority over him - men who could tell him what he could and should do and men whom he had to listen to and obey.
I can see why they let him go. And, honestly, if you were a boss and an employee disrespected you in front of your employees, I dare say you'd get rid of that employee too, or risk being seen as a weak and effective leader.