I thought the US wanted to replace damaged US gear?
Also, don't they have lots of M270 launchers too? They're more protected for more dangerous launch areas.
More launchers with less ammunition could be a good deterrent. You can cover more area and launch less missiles becuase the enemy would just avoid more area. Also they can be useful spares in case of massive attacks.
That, or maybe the purchase also covers ammunition. I expect they come with technical support and maintenance service, so why not supply service?