Amazon Prime is particularly heinous about using dark patterns to confound users into risking forgetting but ultimately you’ve already paid for the month, year
I can confirm this to also be the case with most streaming giants plus the less-giant Shutter
Edit: comments have pointed out some notable exceptions such as services through Apple and HP Instant Ink
If they don’t prorate a refund then they’re stealing from you. Obviously if this is a trial they can cancel immediately if you haven’t paid but otherwise it’s theft.
I agree, but good luck getting anywhere with it if they choose to not refund you.
That’s what charge backs are for.