Actually, the_luna has a point. If you plant a tree peony thinking it's the regular kind, you're in for a bad time. The tree ones need different pruning, they don't die back, and they cost a lot more. Mixing them up means you could kill a plant you spent good money on. It's not just being picky, it's practical.
Get over it? They are not all the same. Tree peonies are woody shrubs that live for decades, while herbaceous peonies die back to the ground each year. The growth habit, care needs, and even bloom times are completely different. Calling them all just "peonies" ignores a huge and important distinction in the plant world. It shows a real lack of basic gardening knowledge.