This was an agreement between local tribes and the Syrian Democratic Forces to limit the already immense destruction of Raqqa and protect civilians that were held as human shields.

If the Syrian Democratic Forces and the US to just say "No survivors!" and release the MOAB they would likely have a very hard time gaining the trust of the local tribes and communities in the future.

"This was a local solution to local issue. Coalition did not fully agree, but respected our partners decision." - https://twitter.com/OIRSpox/status/930166228727066626