Because for better or worse, email has become the defacto identity identifier on the web. If you allow people to grab emails that used to be owned by someone else, you effectively allow them to own their identity on every website where they registered with that email in the past.

And while some other email providers do the same, it's particularly bad with yahoo because they are such a huge and longstanding provider, they have tons of email, some that are a decade old, and a lot owned by people who are not very technical or who don't check their email very often.