There is one good reason: abuse.

Think about how long it took to get email spam under control (and even so it remains a problem). Think about the ongoing problems with spam phone calls. Now pretend you are someone with no technical background, someone who only sees the UI and does not really understand what happens behind it -- you are not seeing spam on closed systems, you are seeing spam on open systems, what are you going to choose? Also keep in mind that there are worse forms of abuse than spam.