Why does `tokio::spawn` require `Send`?

I was reading through Niko's blog recently, and he spoke on how they want async traits to not require Send + 'static for a number of async uses, which sounds good.

But looking at tokio::spawn, it does require Send + 'static, even if you're using the single-threaded runtime. If there's only one thread, why does it need Send? Is this a historical thing they don't want a breaking change for? Or is there some API I'm missing that fits my use-case?