I guess since in many cases you do actually need to roll a dice, like when peeforming a touch or ranged touch spell, people just assume it always happens.
And even in this case. Cure wounds is a spell like any other and it is subject to a will saving throw. So to be correct the pc that was targeted by the spell would indeed roll in order to save from the unintended heal - but thats really just assuming the spell could be used like this, which in my interpretation it cannot.
So again, even if the caster rolls no dice in this case, the target could. I think this leads to people thinking there must always be a roll.
Edit: fix paragraphs
Thats not what I said.
Never played 5e but in 3.5e the target of the spell - not the spellcaster - can roll a dice. The target can perform a will save to reduce healing amount by half.