Still screwing me after I left

After battling with Roger's over their billing practices for 6 months, my contract fortunately expired.  I called a month prior to the expiration to inform them that I was cancelling my service with them.  On the first possible date I went out and got a new phone and a new carrier.  Thirty days later I got yet another bill from Rogers, so I called to get it fixed and they informed me that they had the right to bill me for 30 days after cancelling because I went to another carrier.  How can this be legal???