I'd guess that depends on whether their standards for guilt are more like criminal or civil court.
It's not a matter of the standard of guilt, it's a matter of the authority they have to punish players.
The players are members of a union, and part of their collective bargaining agreement with the league provides for arbitration of disputes over discipline handed down by the league.
Arbitration law and industry custom provide that in order for a member of a union to be bound by a punishment, she must have advance notice of the specific policies she is required to follow and the penalties for violation of those policies.
What that means is that the NFL is not allowed to invoke a vague "conduct detrimental to the league" policy when pointing to what a punishment is based on, nor can they make up a punishment on the spot to fit the specific circumstances of the alleged misconduct. It all must be provided for in the language of the Collective Bargaining Agreement. Otherwise, the NFL has not effectively bargained for the power it seeks to use in punishing the employee.