A malicious recipient can click the link and exploit that their email is now associated to the account of some other person.
But it could be mitigated by "click the link and enter the one time code we gave you at sign-up time". Too much friction? How about "click the link on the same browser you used to sign up, and we'll verify that using a cookie we just set" - functionally equivalent and probably works for 90% of users while the rest can fall back to the one time code.
I've seen a handful of sites do something like this in practice. No idea why it's not more common: presumably most people don't roll their own verification process so if some major web frameworks adopt it we'll eventually see it more widely.
Oh boy. Auth is that thing that looks so easy because you just need to store an md5 password to feel like hackerman. If people actually used existing solutions, web logins wouldn’t be in such dire conditions.
Not all careless stupidity should be attributed to the website admin, however: assholes using random email addresses and phone numbers deserve to be punished, and knowing one's own email addresses is a basic literacy requirement.