That is dependent on the leadership. My musicians were treated with respect and ate well at all private events where food was served. Usually, I negotiated a plate or trip to the same buffet as the guests, plus a couple of drink tickets, and put it in the contract. Once in awhile we had to settle for a "vendor meal" box lunch or the client sprang for pizzas for the band, but those were the exception and only if the money was crazy good. For extra $$$ my players understood they might have to settle for a PBJ or bring their own. Sometimes we had time to run down the street for a decent meal, and because of the price I negotiated, the client paid for it whether they realized it or not. All that and much more is negotiated and contracted in advance by the BL. It's true that if you let the client take advantage and twist you around their finger, they will do it.
Someone made the point earlier that private events can be stressful, and that is true, especially the really deep pocket stuff. Even in the ideal the expectations are high and the details seemingly endless. All of these clients expect the perfect day, regardless of the fee, and they can sometimes be a little over-demanding to the point of unreasonable. Sometimes it stems from ignorance; most of these clients rarely plan parties, don't work with entertainers often, and they can inadvertently paint the entertainment into a very tight corner without realizing it. But in the process of making sure you can do your job without stressing out your players, there is nothing wrong with a gentle primer in hiring a band and outlining some steps the client can take to remove some of the pressure. I would explain to the client that the less pressured we are, and the more the logistics favored us, the better product and dance party we could deliver, and whenever possible those details were outlined in the contract. That usually did the trick, but I have declined to contract with some clients because of unreasonable and inflexible expectations. There is no point agreeing to a bad fit, having a horrible experience, and risking your reputation in the process.