In both vehicles, the primary reason why the non-integrated design of the infotainment screen is suitable is the deferential nature of the rest of the interior.
The Tesla Model 3 prototype, for example, not only lacks an instrument cluster for the driver, but its minimalist design hides any visible air vents and other physical buttons and switches. Consequently, the large infotainment screen becomes the only source of entertainment, information and control for the driver and passengers. As a result, the remainder of the interior is deferential to the infotainment screen, thus validating its tablet style placement in front of the dashboard such that it becomes the primary focus and can be optimally viewed by both the driver and passengers.
The BMW i3 follows a similar principle. The air-vents, media and ventilation controls are functional, rather than fancy, in nature, and thus defer attention towards the infotainment system. In the specific case of the i3, a half length centre console means that there is no traditional division between the driver and passenger, and hence no 'tee' to the interior design. This enables BMW to instead place emphasis upon the horizontal plane of the interior in an effort to accentuate the width of the car. The 'floating' style placement of the infotainment screen, together with its widescreen display ratio, combines with the lengthwise orientation of the wood trim to further highlight the width of the dashboard.
Fundamentally, automotive manufacturers should place the infotainment display such that it respects the intent and the inherent architecture of the interior. Where this architecture is a 'T' shape, the infotainment system as a rule of thumb should be located cleanly in either the dashboard (horizontal axis) or within the centre console (vertical axis), possibly by using a portrait orientation. In contrast, if the interior is deferential to the infotainment system, the display should generally be located at a focal point where it can be optimally viewed by all occupants of the vehicle.