Since it is a "free" call, people think they can call the number to get general Disney information. Disney didn't do away with their 800 numbers for no reason. It got expensive for them. Plus members pay for these calls out of our annual fees.
Non-members were calling MS (some multiple times) with requests to verify points reservations, and to ask questions, add requests, and make changes to rented reservations. etc. Since Member Services can/will only talk to members and number is widely available to members, there was really no need to publish it here and put MS in a position to refuse to talk to the non-members.
Now that non-members can verify their rented reservations via www.mydisneyvacation.com, the restriction is probably not as important as it may have been in the past.