The angled objective lens is part of the design.  All red dot sights have to have a bit of an angle on the objective to allow for the light from the LED emitter to bounce off the inside surface and be reflected to the shooter.

The Holosun design is a bit steeper than many and this allows for the emitter to have an extremely low profile so that it does not intrude into the inside of the tube and obstruct your view.   The angle also reduces reflection to your target.