Let's assume that the coach did not explain she would be pitching every game. So what? Did the parent(s) ask?
Amen.
This applies to all pickup situations, not just pitching.
Any time there is a pickup player, whether it's a pitcher or not, there should be communication between coach and parent about how the coach plans to use the player. Although preferable, it's not necessary that the coach be very specific, or make any promises. What's important is that coach and parent and player are on the same page going in. It's remarkable how often parents get mad when their child doesn't play the role they expected as a pickup player. And 9 times out of 10, they didn't ask ahead of time. They just made assumptions.
And this communication is the responsibility of both coach and parent. One has no right to blame the other.