since i have a free trial on apple tv i wanted to see a film they asked me to rent it or to buy it why?

The subscription only covers Apple-original content; click here for the list of it. Items which show an individual price aren't part of either paid or trial memberships and cost the same regardless of status.

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Even then, some Apple Original content is not currently 'free'. Apple has been releasing films in theaters for a while now. they will eventually be free on TV+, but not all are yet.