Hall and Law & Order

How does L&O affect the audience in terms of ideology?

Hall would definitely mention that the show is encoded with the dominant view of the criminal justice system (in the show's case, the producers' view). It doesn't matter that the producers may have never worked in an American police department or even as a lawyer or judge -- they write how they think it works, not necessarily how it really works.
When the audience sees L&O, they will decode the message and take it one of three ways, depending on their background.
One group (the dominant reading) will believe that L&O's portrayal is completely accurate. They will not question the show. They believe that the actors are playing police officers just as police officers act in reality, that lawyers and judges behave just as they do on the show, and even the actors playing criminals play their roles realistically.
Another group (the negotiated reading) will accept that generally, L&O is accurate - but these viewers may have personal experiences with the justice system that don't fully align with L&O. They thus adjust L&O's code to fit their interests, despite the contradictions that inherently arise.
The final group will reject L&O completely. They may work as a lawyer in NYC, or maybe as a cop or have been arrested before -- or even have simply done a good deal of research in the field. Whatever the circumstance, they do not believe that L&O is accurate. They understand the preferred reading but still reject it.