I don't think the reason is to prevent accidental/destructive deletion. There are better ways to do that such as providing a grace period; allowing users to download their account data for a future re-signup; and two-factor authentication.

It's more likely that the revenue earned by the service is heavily dependent on building a social profile. A properly deleted (without a trace) account reduces the profile accuracy. I also wouldn't rule out the almost too obvious government surveillance requirement.