The last time I checked, the NFL is a private organization and you can be punished for any violations of their regulations, regardless of what the law states. It might not be legal punishment under the justice system, but in the military we call it non-judicial or administrative punishment.
Think about it - you can get fired at work for doing something like leering at a female co-worker, showing up late, etc. But what law exists in any public code or The Constitution that says you go to jail for staring at women or being late to work? None. This isn't a dictatorship, and contrary to what many fans think, the NFL is certainly not a democracy.
For further reading, here is a snippet from an article detailing how Roger Goodell views player conduct, and an excerpt from the letters he sent to Chris Henry and Adam Jones:
Source: ESPN.com Article
Based on that, you be the judge on if Roethlisberger's conduct has "brought embarrassment and ridicule" upon himself, the Pittsburgh Steelers, and the NFL, along with all of the rest of Goodell's criteria. Like it or not, the facts show that a double-standard exists.