The situation that you face is that you cancelled a one year annual subscription as opposed to a month-by-month subscription. It is your choice when you subscribe. The annual subscription costs less per month than the month-by-month subscription, but the month-by-month subscription carries no cancellation fee. The annual subscription is similar to a lease on an apartment – you commit to the full term. Adobe does allow you to cancel the annual subscription, but the cancellation fee effectively pays the difference between what you paid on a monthly basis with the annual subscription versus what you would have paid for the month-by-month subscription. In other words, you are paying back the discount you got for the months you used in the annual subscription. Adobe is absolutely not holding you to pay for all the months of the annual subscription remaining!
We don't really see anything unfair about that. What you need to do is read the terms and conditions before you commit to any subscription, whether from Adobe, Microsoft, or any other software vendor with subscription-based software products.