When I watch the theatrical cut of Justice League on HBO Max, there are no black bars.
For example, this image makes no sense. I found it on a site trying to compare ratios. Let's say those yellow lines are like how Justice League theatrical cut is currently on HBO Max, with no black bars. Let's just say those yellow bars are the shape of your TV since that's how it looks on HBO Max with no top or bottom black bars. Those blue bars would make you believe your TV suddenly grew beyond the boundaries of the yellow bars. In reality, those blue bars on top and bottom would have to be squeezed down to fit the height of the yellow bars aka the height of your TV, hence the image is shrinking. Now you have a smaller image with black bars on the side. Sure, it may help distinguish short versus taller better, but it's also making certain things smaller. It may look great on a giant IMAX screen, but not your normal 40-60 inch TV at home.