they aren't, but tesla's current market is the new adopters, and they could become another brand to the mix, but at the end of the day the real electric cars that we'll be driving will be mainly made by the same people that are making our petrol and diesel cars currently.
While the "powered by Tesla" tag may end up being a great sales pitch (perhaps the AMG/McLaren of the electric car world) by the big companies I wonder if they have shot themselves in the foot by open sourcing their patents a couple of years ago.
Why use tesla parts when you can just use the underlying tech for "free"?