Not exactly sure. My understanding, for what it is worth, is that butterfly bushes can take a very hard pruning.

I have butterfly bushes on my very steep, very large hill. I am in zone 6 and cut mine back hard in the spring (down to 6" tall). It is very drastic, but I do it for 2 reasons: in the years I've been too lazy to cut them back the weight of the plant leaning down the hill will start to pull the bush out of the ground; and the branches grow wildly out of control and don't look nearly as nice (smaller blooms) if I don't cut them back. It's just my opinion, but I'd cut them back. You might want to check with your landscaper.

I have butterfly bushes on a hill in my backyard that get 10' to 15' tall and as wide. Once a year I have my gardener cut them down to the ground with a chain saw and within 2 months they are 4' tall and beautiful. You can be brutal. They are very tough once established.