I hope we haven't internalized the idea that corporations should be treated the same as people.
There's essentially no difference $3M and $300M fine against most individuals, but $3M means very little to Tesla. If you want Tesla's behavior to change - and other automakers take notice and not repeat the behavior - then the fine must be meaningful to them.
That's another difference - fining an indivisible is not going to change risks much, the individual's behavior changing is not that meaningful compared to Tesla's behavior changing. And it's not like a huge fine is gonna make a difference in other drivers deciding to be better, whereas other automakers will notice a huge fine.