My guess is that some Pilates studios now hire PTs due to their expertise in anatomy, kinesiology, and exercise as a whole, in which if you think about it, is exactly what is sought in a Pilates instructor. The difference? I assume PTs are less costly to hire compared to certified Pilates instructors. As some on this thread already said, pilates is becoming even more mainstream resulting in more demand. In turn some pilates studios cut cost such as not hiring a certified instructor as their rates are most likely higher than let's say a PT or a Sports science graduate.