Frankly this has never made sense to me either. The whole "you can cut out the background in Photoshop" thing is meant as a way of determining whether a background is simple or complex. If you can easily cut it out, then it's simple, otherwise it's complex. It shouldn't be taken literally, as in "transparent backgrounds aren't simple because they're already cut out".
Artists who take commissions often have different prices for simple versus detailed backgrounds. I look at it like that - simple background means the picture isn't a full scene with a full background, it's just a character in empty (or nearly empty) space.
On a practical level, it's very hard to tag simple background because you have to verify that it's not transparent first. This makes tag scripting difficult. It also makes automatic tagging via things like DeepDanbooru impossible, since transparent backgrounds usually get converted to white.