Essentially, it comes down to fraud prevention and usage.
Fraud Prevention: Due to the way our monitoring platform works, if hundreds of accounts suddenly appeared to monitor the same website, that could lead to a degredation of service to that website under certain circumstances. Imagine 500 accounts monitoring the same website at one minute intervals from 5 different locations -- thats 2500 connections per minute as a result of a malicious user.
Usage: While we do give all new clients $25 in account credit, that may last some customers several months, and it may last some customers an hour. It all depends on usage. Having a card on file allows for continuation of service instead of suspension of service.