In-house AV companies are vendors to the hotel that pay a commission to the facility to be in-house. You are not required by any hotel to use an in-house audio visual company.
While many of these companies are very reputable, it is important to understand the potential challenges with using them. First, in-house AV companies pay a very high commission to the hotel in order to be located within the facility. This commission can often go higher than 50% on the dollar. That means in order for them to ensure profits, they must dramatically increase the rates that they charge you, the customer, to make up for the commission they pay the hotel.