Merchant processors are required by law to verify a person's ACH information (bank account) as part of their antifraud efforts and initiatives. This is a rule from the National Automated Clearing House (Nacha), which requires ACH originators of web debit entries use a "commercially reasonable fraudulent transaction detection system" to screen web debits for fraud. The new rule supplements that screening requirement, making it explicit that "account validation" be part of that detection system.

For a more detailed explanation of why this is mandatory, refer to this document here.