The serious answer; Nothing, at all. I totally understand why they do it too, the money involved is of a seriously good value.
My opinion; Everything in terms of aesthetics. If you're trying to perceive a theme of a certain ride, wether or not that may be a big factor of the ride, or even just an under-lying theme, it doesn't matter. I don't understand how you can portray a certain theme and narrative when you're plastering Hair Products on the side of the cars. It's like, "Wow, that's a cool mine train, oh, I forgot, I need to go to the shop to buy some Shampoo". It just doesn't fit at all.
A lot of Theme Parks are trying to make their guests escape their normal lives, just for a few hours and enjoy the thrills and spills of their rides. Advertising outside products will suspend that disbelief, even just for a few minutes, but it's defeating the objective of the parks aims.