Can’t they just drop in at a local mosque and pray with the local imam?
No. Muslim servicemen have as much a right to clergy to help them through their spiritual beliefs as, say, Presbyterian soldiers. If you're going to provide Evangelical chaplains, you have to provide Islamic ones.
Pluralism is a good thing. It's what allows us to agree to worship in our own way, and then get back to work defending this country.
I'm not sure that is always allowed, especially in a combat zone. In many cases, especially when a unit is on the move, that would be impractical or potentially dangerous to the serviceman.
We're a religiously pluralistic nation and there are Muslims serving in our military. I see nothing wrong with the military trying to meet their religious needs, as they do for other religions.