I think it's worth making clear, because in many walks of life, a trial period, subscription, whatever, comes with strings attached, like you have to buy six issues or buy six bottles or subscribe for six months.
I, too, imagine that one will be able to cancel during the trial period with no obligation to pay for a subscription, but there is nothing from Apple to that effect. Ergo, it is wise to err on the side of caution, and check the fine print before signing up for the trial period of Apple Music. As Apple have not released any fine print detailing this, one has to assume that one can only cancel after paying for a subscription, as Apple have stated.