Unfortunately that's a legal requirement. Credit card regulations in the US require that cardholders can revoke authorization for charging their card at any time. It wasn't until I began running a business that I realized just how many protections credit card holders are given. As a consumer I appreciate it, but as a business owner it's very annoying to have customers remove their card on file or dispute a payment.
I'm not a lawyer and this is definitely not legal advice: You could get a customer to sign a contract stating that their card will be charged $X for Y months, but then you'd have to write down their card info and keep it securely in a safe or locked file cabinet, then manually enter the card number whenever you want to charge them. The customer could still dispute a payment, and you'd probably wind up going to court because (in my experience) banks don't care about contracts when it comes to disputes.
I do something like that for big parties where I could be on the hook for hundreds, if not thousands, of dollars if the party cancels at the last minute. It's a pain to do, which is why I don't do it for smaller parties. In those cases if I get an email that a customer has unlinked their card then I immediately contact them to get it resolved.
