I wonder if the logic is to do with it being Burns still in? We're now desperate for Burns to bat long, so don't really want scoring pressure on him. Burns is left hand bat as is Stokes, so perhaps Buttler as a right hander is seen as a) ensuring they don't have 2 left handers to bowl at, and b)...
I get what you're saying, but in test cricket whether you open, or coming in 4, 5 or 6, there are times you have to protect your wicket. If Roy wants for be a Test batsmen, its no good saying he just has to be aggressive, his batting attributes have to adjust to conditions, score, time etc...