If Tesla didn't want Lemonade to provide this, they could block them.
Strategically, Tesla doesn't want to be an insurer. They started the insurance product years ago, before Lemonade also offered this, to make FSD more attractive to buyers.
But the expansion stalled, maybe because the state bureaucracy or maybe because Tesla shifted priority to other things.
In conclusion: Tesla is happy that Lemonade offers this. It makes Tesla cars more attractive to buyers without Tesla doing the work of starting an insurance company in every state.