The hot shoe on your 750D is reporting the presence of the accessory attached to the hot shoe. Even though the accessory is not conductive, the pressure it places on the spring under the side rail on the right side of the hot shoe (as viewed from behind the camera) is depressing a switch under the hot shoe to let the camera know something is attached.
I wouldn't blame Canon, they've been doing it that way as long as the addition of popup flashes to SLRs. The blame should lie squarely on the shoulders of whoever designed your accessory. They should have known better. I have seen accessories intended for mounting on the hot shoe that only have one edge that fits below the outer rail on the opposite side of the shoe from the spring switch.
The only workaround I can think of that even might work is to pop the built-in flash up before attaching the accessory to the hot shoe.