It's protection against bulk account creation. Getting a phone number costs money, so requiring a phone number makes it expensive to get 1000 accounts.
Phones are ubiquitous enough that very few customers have to pay this cost.
Obviously, this reasoning doesn't (and shouldn't) extend to using phones for authentication.
Yeah, after writing the article i also realized i dont mind verification actually. As long as they support whatever carrier i currently have. Its authentification that i bother with. (As in i have a phone number, i want internet on my phone, but i change it often monthly)
Also services like Twitter that allow any rented SMS service and multiple accounts for each number this seems not be the reason why they ask for a number.
Phones are ubiquitous enough that very few customers have to pay this cost.
Obviously, this reasoning doesn't (and shouldn't) extend to using phones for authentication.