How are transit providers supposed to do source filtering on each other? Where Centurylink peers with Verizon, where is Centurylink supposed to get that source

How are transit providers supposed to do source filtering on each other? Where Centurylink peers with Verizon, where is Centurylink supposed to get that source filtering table from? What if a Centurylink customer (who has their own AS) dumps them for Verizon? This problem repeats with the Tier-2 transit carriers, and Tier-3, and so on. You're expecting an exponential number of source filter updates every time somebody moves their route announcement from one provider to another. This does not scale. Source filtering can only really be implemented when you're taking traffic from a network for which you are the only uplink. If El Badguy (tm) is on one of these networks there really isn't a practical place to source-filter them.