I had a LAX-MEX-GRU-MEX-LAX itinerary in business class. They cancelled my outbound flights and now want to charge me the fare difference to rebook on another date. It was a discounted business class fare and the fare difference is now thousands of dollars. They claim the cancellation was due to coronavirus and therefore they are allowed to charge the difference in fare. The attendant was extremely rude on the phone.

Are they allowed to charge the fare difference if they cancelled my original flight? I have no interest in getting a voucher and I understand they are not giving full refunds either.