Yes you are correct in saying that Aerospace & Military are Exempt from RoHS law....BUT...
1. Electronic Suppliers and Manufacturers are global - Europe & China Market.
2. The Military and Aerospace are about 1% of the World Consumer Market
Most of Aerospace and Military procure electronics and surplus parts through electronic suppliers and manufacturer's globablly, which then affect COTS (commercial-off-the-shelf) parts.
To Maintain Interest in Europe the electronics industry is starting to move over to non-lead solders.
There has been many case studies has been done on this issue and tests that can be done to determin and mitagate the effects of tin-plate whiskering, but currently there is no technique that guarantees the DoD requires for high-reliability systems "except the addition of 3% or more lead to tin.
There is no direct replacement or substitution that "gets the lead out," Lead-Free electronic components are simply unavoidable.
See
Lead-free solder: A train wreck in the making - Military & Aerospace Electronics
And
Tin-whisker failure problem from lead-free solder in military electronics is target of Air Force research job - Military & Aerospace Electronics
For more info.