The term "beach read" has long been used to describe a completely frivolous book with no literary value. The connotation is that it is best enjoyed on the beach while on vacation, when your brain can be turned off.
The term has often been used to describe romance novels and other genres, such as suspense and mystery, that are considered purely for entertainment.
I think we have been misunderstanding the function of beach reads. It is not that they are good for beach reading because they will not force us to think. Rather, they enhance our vacation by allowing us to escape our everyday lives and vicariously experience adventure and romance.
It was those Corona "find your beach" commercials that finally helped me make the connection. Just as Corona is supposed to help you escape the stresses of ordinary life and imagine you are on a sunny beach rather than a downtown watering hole after work, a beach read will transport you to another world and allow you to imagine a more exciting and romantic life for a little while.
I think that is why I enjoy historical romances so much more than contemporary romances. Contemporary romances deal with real-world stresses and emotions and do not allow the same level of escapism for me.
My beach is generally 19th-century England.