This is not about Tilia getting copies of passports and utility bills; they already have much more than they ever needed. They have everything you ever sent to Linden Research, Inc.
Tilia is a growing, very important service for Linden Research's future. It needs as many users as possible.
SL is a very old service, winding down. It has many users.
Forcing each and every SL user - independent of their US$ balance or withdrawal history - to become a Tilia user is a very sensible business decision.