It's not their fault you didn't read the conditions of your subscription carefully enough.
You agreed to an annual subscription, which means you pay less each month, compared to a monthly subscription.
In exchange, you're obliged to the entire yearly term.
If you then cancel that subscription halfway in the middle of the term it's only fair they charge you the difference between what you paid and what you would have paid if you took a monthly subscription.
Otherwise, everyone will only subscribe to the annual plan and pay less, no matter for how long they plan on using it.