Zipcar Scam Change

Today Zipcar sent me an e-mail stating:

Thank you for being a Zipcar member. We’re writing to let you know we’re moving your account to our $7 standard monthly plan and retiring your previous membership plan. This change should be reflected in your next billing cycle, but it may be a few cycles before it shows up on your account.

That is, Zipcar is automatically changing me from a $0/month plan to $7/month plan. And they provide no "Cancel my account" feature on their site.

Is this even legal?