BMW's particular tuning arrangement is more of a support agreement with Dinan.

Dinan develops the modification as an aftermarket "accessory," and BMW agrees to support their parameters, allowing people to undergo the modification while maintaining full warranty coverage.

Just like if I cranked the boost controller on a new turbo gas BMW.

I agree on this aspect of it. If you look at it as a warranty issue, it's not that offensive. However, if they are locking you out of doing your own modifications while selling this it starts to get evil.

I'm actually surprised more people haven't tried to 'tune' the current electric cars this way. Cooking batteries, controllers, motor windings etc.

They haven't?