When you initially agreed to the TOS, there was a clause that said Venmo/PayPal can modify the terms whenever they want:

"We may revise this user agreement and any of the policies listed above from time to time. The revised version will be effective at the time we post it, unless otherwise noted. If our changes reduce your rights or increase your responsibilities we will provide notice to you of at least 21 days. We reserve the right to amend this agreement at any time without notice, subject to applicable law. By continuing to use our services after any changes to this user agreement become effective, you agree to abide and be bound by those changes. If you do not agree with any changes to this user agreement, you may close your account."

https://venmo.com/legal/us-user-agreement/

Every TOS you've ever agreed to has a clause like this. These agreements are extremely one-sided.