Hamas would turn off the missile system and prevent it's people from seeking shelter (as it did during the last conflict) or worse use their people as usual as human shields. Hamas is interested in offensive weapons only (preferably a guided missile that targets Jewish pre-schools) not in defensive ones. Hamas is after all a mass murdering terrorist organization.
U.S. Democratic Senator Merkley airs support for Iran nuclear deal (Reuters)
from the article: U.S. Congress gives Obama okay to fund Israel rocket defense