If your child asks you if he can paint a picture, and you say yes, but specify that he has to clean up the mess when he is done, are you going to then clean up the mess for him because when it's time to do the actual work of clean up, he whines about having to do his part, which was stated beforehand?
Situations look muddy when there are multiple incompatible goals. The goals above are 1) giving a child an opportunity to be creative and 2) to learn to stick to promises.
Personally I think the most important goal in that scenario is for the child to create. If they want to be creative enough to agree to clean up then they must want it pretty badly.
If the mother is putting a barrier of "you must clean up afterwards" between the child and an outlet for creativity, if the creative juices aren't flowing fiercely enough then they'll choose not to be creative.
So what's been lost? There are plenty of opportunities to experience both sides of promises: promising and being promised. I don't think creativity should be sacrificed for a lesson in being true to your word.
My daughter does treat her promises seriously because I've made a point to treat my promises seriously. We've talked about what it means to lose confidence when a friend breaks a promise and how it's hard to regain that confidence. She knows the consequences and doesn't want to lose my confidence nor do I want to lose her confidence in me. So we're honest with each other. But that means that I shouldn't pressure her into making promises that it will be very difficult for her to follow through on, or that might interfere with something more important. I don't want her ever to choose not to do something because she is avoiding some made up consequence.
My job is to help her explore the world. Part of that is setting up and cleaning up after her creativity. I do ask her to help. And now that she's 12 she does do it willingly and can do enough to actually be helpful. Before that, though it seemed a minor thing to me, it really was a major task to her and too big of a burden that got in the way of being creative.